[PATCH 0/1] Limit client version advertisements

[PATCH 0/1] Limit client version advertisements

[Josh Steadmon]

As discussed in [1], clients will incorrectly advertise support for
protocol version 2 even when the service in question does not have a v2
implementation. This patch sets maximum protocol versions for
git-receive-pack, git-upload-archive, and git-upload-pack.

[1]: https://public-inbox.org/git/20180927012455.234876-1-steadmon@google.com/

Josh Steadmon (1):
  protocol: limit max protocol version per service

 connect.c     | 11 ++++-------
 protocol.c    | 13 +++++++++++++
 protocol.h    |  7 +++++++
 remote-curl.c | 11 ++++-------
 4 files changed, 28 insertions(+), 14 deletions(-)

-- 
2.19.0.605.g01d371f741-goog

[PATCH 1/1] protocol: limit max protocol version per service

[Josh Steadmon]

For services other than git-receive-pack, clients currently advertise
that they support the version set in the protocol.version config,
regardless of whether or not there is actually an implementation of that
service for the given protocol version. This causes backwards-
compatibility problems when a new implementation for the given
protocol version is added.

This patch sets maximum allowed protocol versions for git-receive-pack,
git-upload-archive, and git-upload-pack.

Previously, git-receive-pack would downgrade from v2 to v0, but would
allow v1 if set in protocol.version. Now, it will downgrade from v2 to
v1.

Signed-off-by: Josh Steadmon <steadmon@google.com>
---
 connect.c     | 11 ++++-------
 protocol.c    | 13 +++++++++++++
 protocol.h    |  7 +++++++
 remote-curl.c | 11 ++++-------
 4 files changed, 28 insertions(+), 14 deletions(-)

diff --git a/connect.c b/connect.c
index 94547e5056..4a8cd78239 100644
--- a/connect.c
+++ b/connect.c
@@ -1228,14 +1228,11 @@ struct child_process *git_connect(int fd[2], const char *url,
 	struct child_process *conn;
 	enum protocol protocol;
 	enum protocol_version version = get_protocol_version_config();
+	enum protocol_version max_version;
 
-	/*
-	 * NEEDSWORK: If we are trying to use protocol v2 and we are planning
-	 * to perform a push, then fallback to v0 since the client doesn't know
-	 * how to push yet using v2.
-	 */
-	if (version == protocol_v2 && !strcmp("git-receive-pack", prog))
-		version = protocol_v0;
+	max_version = determine_maximum_protocol_version(prog, version);
+	if (version > max_version)
+		version = max_version;
 
 	/* Without this we cannot rely on waitpid() to tell
 	 * what happened to our children.
diff --git a/protocol.c b/protocol.c
index 5e636785d1..1c553d8b99 100644
--- a/protocol.c
+++ b/protocol.c
@@ -79,3 +79,16 @@ enum protocol_version determine_protocol_version_client(const char *server_respo
 
 	return version;
 }
+
+enum protocol_version determine_maximum_protocol_version(
+		const char *service, enum protocol_version default_version)
+{
+	if (!strcmp(service, "git-receive-pack"))
+		return protocol_v1;
+	else if (!strcmp(service, "git-upload-archive"))
+		return protocol_v1;
+	else if (!strcmp(service, "git-upload-pack"))
+		return protocol_v2;
+
+	return default_version;
+}
diff --git a/protocol.h b/protocol.h
index 2ad35e433c..eabc0c5fab 100644
--- a/protocol.h
+++ b/protocol.h
@@ -31,4 +31,11 @@ extern enum protocol_version determine_protocol_version_server(void);
  */
 extern enum protocol_version determine_protocol_version_client(const char *server_response);
 
+/*
+ * Used by a client to determine the maximum protocol version to advertise for a
+ * particular service. If the service is unrecognized, return default_version.
+ */
+extern enum protocol_version determine_maximum_protocol_version(
+		const char *service, enum protocol_version default_version);
+
 #endif /* PROTOCOL_H */
diff --git a/remote-curl.c b/remote-curl.c
index fb28309e85..028adb76ae 100644
--- a/remote-curl.c
+++ b/remote-curl.c
@@ -344,6 +344,7 @@ static struct discovery *discover_refs(const char *service, int for_push)
 	int http_ret, maybe_smart = 0;
 	struct http_get_options http_options;
 	enum protocol_version version = get_protocol_version_config();
+	enum protocol_version max_version;
 
 	if (last && !strcmp(service, last->service))
 		return last;
@@ -360,13 +361,9 @@ static struct discovery *discover_refs(const char *service, int for_push)
 		strbuf_addf(&refs_url, "service=%s", service);
 	}
 
-	/*
-	 * NEEDSWORK: If we are trying to use protocol v2 and we are planning
-	 * to perform a push, then fallback to v0 since the client doesn't know
-	 * how to push yet using v2.
-	 */
-	if (version == protocol_v2 && !strcmp("git-receive-pack", service))
-		version = protocol_v0;
+	max_version = determine_maximum_protocol_version(service, version);
+	if (version > max_version)
+		version = max_version;
 
 	/* Add the extra Git-Protocol header */
 	if (get_protocol_http_header(version, &protocol_header))
-- 
2.19.0.605.g01d371f741-goog

Re: [PATCH 1/1] protocol: limit max protocol version per service

[Stefan Beller]

On Tue, Oct 2, 2018 at 3:00 PM Josh Steadmon <steadmon@google.com> wrote:
>
> For services other than git-receive-pack, clients currently advertise
> that they support the version set in the protocol.version config,
> regardless of whether or not there is actually an implementation of that
> service for the given protocol version. This causes backwards-
> compatibility problems when a new implementation for the given
> protocol version is added.
>
> This patch sets maximum allowed protocol versions for git-receive-pack,
> git-upload-archive, and git-upload-pack.
>
> Previously, git-receive-pack would downgrade from v2 to v0, but would
> allow v1 if set in protocol.version. Now, it will downgrade from v2 to
> v1.

But does git-receive-pack understand v1?
As to my understanding we have not even defined v1
for push (receive-pack) and archive --remote (upload-archive).
v1 is only known to fetch (upload-pack).

> +enum protocol_version determine_maximum_protocol_version(
> +               const char *service, enum protocol_version default_version)
> +{
> +       if (!strcmp(service, "git-receive-pack"))
> +               return protocol_v1;
> +       else if (!strcmp(service, "git-upload-archive"))
> +               return protocol_v1;

so I would think these two would be _v0.
... goes and checks ...
aa9bab29b8 (upload-pack, receive-pack: introduce protocol version 1,
2017-10-16) seems to actually teach v1 to receive-pack as well,
but upload-archive was completely off radar, so I think returning
(v1, v0, v2 in the order as in the code) would make sense?

Asides from this, I thought there was a deliberate decision
that we'd want to avoid a strict order on the protocol versions,
but I could not find prior discussion on list to back up this claim. :/

For example we'd go with e.g. enums instead of integers
for version numbers, as then some internal setup could
also have things like protocol_v2018-10-02 or protocol_vWhatever;
some protocol version may be advantageous to the client, some to
the server, and we'd need to negotiate the best version that both
are happy with. (e.g. the server may like version 0, 2 and 3, and
the client may like 0,2,4 as 3 is bad security wise for the client,
so both would negotiate to 2 as their best case)

From a maintenance perspective, do we want to keep
this part of the code central, as it ties protocol (as proxied
by service name) to the max version number?
I would think that we'd rather have the decision local to the
code, i.e. builtin/fetch would need to tell protocol.c that it
can do (0,1,2) and builtin/push can do (0,1), and then the
networking layers of code would figure out by the input
from the caller and the input from the user (configured
protocol.version) what is the best to go forward from
then on.

But I guess having the central place here is not to
bad either. How will it cope with the desire of protocol v2
to have only one end point (c.f. serve.{c,h} via builtin/serve
as "git serve") ?

Stefan

Re: [PATCH 1/1] protocol: limit max protocol version per service

[Josh Steadmon]

On 2018.10.02 15:28, Stefan Beller wrote:
> On Tue, Oct 2, 2018 at 3:00 PM Josh Steadmon <steadmon@google.com> wrote:
> >
> > For services other than git-receive-pack, clients currently advertise
> > that they support the version set in the protocol.version config,
> > regardless of whether or not there is actually an implementation of that
> > service for the given protocol version. This causes backwards-
> > compatibility problems when a new implementation for the given
> > protocol version is added.
> >
> > This patch sets maximum allowed protocol versions for git-receive-pack,
> > git-upload-archive, and git-upload-pack.
> >
> > Previously, git-receive-pack would downgrade from v2 to v0, but would
> > allow v1 if set in protocol.version. Now, it will downgrade from v2 to
> > v1.
> 
> But does git-receive-pack understand v1?
> As to my understanding we have not even defined v1
> for push (receive-pack) and archive --remote (upload-archive).
> v1 is only known to fetch (upload-pack).
> 
> > +enum protocol_version determine_maximum_protocol_version(
> > +               const char *service, enum protocol_version default_version)
> > +{
> > +       if (!strcmp(service, "git-receive-pack"))
> > +               return protocol_v1;
> > +       else if (!strcmp(service, "git-upload-archive"))
> > +               return protocol_v1;
> 
> so I would think these two would be _v0.
> ... goes and checks ...
> aa9bab29b8 (upload-pack, receive-pack: introduce protocol version 1,
> 2017-10-16) seems to actually teach v1 to receive-pack as well,
> but upload-archive was completely off radar, so I think returning
> (v1, v0, v2 in the order as in the code) would make sense?

I believe that git-upload-archive can still speak version 1 without any
trouble, but it at least doesn't break anything in the test suite to
limit this to v0 either.


> Asides from this, I thought there was a deliberate decision
> that we'd want to avoid a strict order on the protocol versions,
> but I could not find prior discussion on list to back up this claim. :/
> 
> For example we'd go with e.g. enums instead of integers
> for version numbers, as then some internal setup could
> also have things like protocol_v2018-10-02 or protocol_vWhatever;
> some protocol version may be advantageous to the client, some to
> the server, and we'd need to negotiate the best version that both
> are happy with. (e.g. the server may like version 0, 2 and 3, and
> the client may like 0,2,4 as 3 is bad security wise for the client,
> so both would negotiate to 2 as their best case)

Is there a method or design for advertising multiple acceptable versions
from the client? From my understanding, we can only add a single
version=X field in the advertisement, but IIUC we can extend this fairly
easily? Perhaps we can have "version=X" to mean the preferred version,
and then a repeatable "acceptable_version=Y" field or similar?


> From a maintenance perspective, do we want to keep
> this part of the code central, as it ties protocol (as proxied
> by service name) to the max version number?
> I would think that we'd rather have the decision local to the
> code, i.e. builtin/fetch would need to tell protocol.c that it
> can do (0,1,2) and builtin/push can do (0,1), and then the
> networking layers of code would figure out by the input
> from the caller and the input from the user (configured
> protocol.version) what is the best to go forward from
> then on.

I like having it centralized, because enforcing this in git_connect()
and discover_refs() catches all the outgoing version advertisements, but
there's lots of code paths that lead to those two functions that would
all have to have the acceptable version numbers plumbed through.

I suppose we could also have a registry of services to version numbers,
but I tend to dislike non-local sources of data. But if the list likes
that approach better, I'll be happy to implement it.


> But I guess having the central place here is not to
> bad either. How will it cope with the desire of protocol v2
> to have only one end point (c.f. serve.{c,h} via builtin/serve
> as "git serve") ?

I'm not sure about this. In my series to add a v2 archive command, I
added support for a new endpoint for proto v2 and I don't recall seeing
any complaints, but that is still open for review.

I suppose if we are strict about serving from a single endpoint, the
version registry makes more sense, and individual operations can declare
acceptable version numbers before calling any network code?



Thanks for the review,
Josh

Re: [PATCH 1/1] protocol: limit max protocol version per service

[Stefan Beller]

On Wed, Oct 3, 2018 at 2:34 PM Josh Steadmon <steadmon@google.com> wrote:

>
> I believe that git-upload-archive can still speak version 1 without any
> trouble, but it at least doesn't break anything in the test suite to
> limit this to v0 either.

ok, let me just play around with archive to follow along:

Running the excerpt that I found in one of the tests in t5000

    GIT_TRACE_PACKET=1 git archive --remote=. HEAD >b5.tar

(whoooa ... that spews a lot of text into my terminal, which makes
sense as transported tar files unlike packets starting with PACK are
not cut short; so onwards:)

    $ git init test && cd test
    $ GIT_TRACE_PACKET=1 git archive --remote=. HEAD >b5.tar
    ...
    git< \2fatal: no such ref: HEAD
    ...
    fatal: sent error to the client: git upload-archive: archiver died
with error
    remote: git upload-archive: archiver died with error

This sounds similar to a bug that I have on my todo list for
clone recursing into submodules. Maybe we need to talk
about HEAD-less repositories and how to solve handling them
in general. Usually it doesn't happen except for corner cases like
now, so:

    $ git commit --allow-empty -m "commit"
    [master (root-commit) 24d7967] commit
    $ GIT_TRACE_PACKET=1 git archive --remote=. HEAD >b5.tar
15:28:00.762615 pkt-line.c:80           packet:          git> argument HEAD
15:28:00.762704 pkt-line.c:80           packet:          git> 0000
15:28:00.766336 pkt-line.c:80           packet:          git> ACK
15:28:00.766428 pkt-line.c:80           packet:          git> 0000
15:28:00.766483 pkt-line.c:80           packet:          git< ACK
15:28:00.766508 pkt-line.c:80           packet:          git< 0000
15:28:00.767694 pkt-line.c:80           packet:          git< \2
15:28:00.767524 pkt-line.c:80           packet:          git< argument HEAD
15:28:00.767583 pkt-line.c:80           packet:          git< 0000
remote: 15:28:00.767524 pkt-line.c:80           packet:          git<
argument HEAD
remote: 15:28:00.767583 pkt-line.c:80           packet:          git< 0000
15:28:00.768758 pkt-line.c:80           packet:          git> 0000
15:28:00.770475 pkt-line.c:80           packet:          git<
\1pax_global_header
    ... \0\0\0\0\0\0\0\0\0\0\0\0\ ...
 # not too bad but the tar file contains a lot of zeros
    $

Ah I forgot the crucial part, so

    $ GIT_TRACE_PACKET=1 git -c protocol.version=1 archive --remote=.
HEAD >b5.tar
15:33:10.030508 pkt-line.c:80           packet:          git> argument HEAD
...

This pretty much looks like v0 as v1 would send a "version 1", c.f.

    $ GIT_TRACE_PACKET=1 git -c protocol.version=1 fetch .
15:34:26.111013 pkt-line.c:80           packet:  upload-pack> version 1
15:34:26.111140 pkt-line.c:80           packet:        fetch< version 1
....


>
> Is there a method or design for advertising multiple acceptable versions
> from the client?

I think the client can send multiple versions, looking through protocol.c
(and not the documentation as I should for this:)

  /*
   * Determine which protocol version the client has requested.  Since
   * multiple 'version' keys can be sent by the client, indicating that
   * the client is okay to speak any of them, select the greatest version
   * that the client has requested.  This is due to the assumption that
   * the most recent protocol version will be the most state-of-the-art.
   */
    ...
    const char *git_protocol = getenv(GIT_PROTOCOL_ENVIRONMENT);
    string_list_split(&list, git_protocol, ':', -1);
    ...
    for_each_string_list_item(item, &list) {
        ...
        if (skip_prefix(item->string, "version=", &value))
            ...

in determine_protocol_version_server which already had the client
speak to it, so I think at least the server can deal with multiple versions.

But given that we transport the version in env variables, we'd
need to be extra careful if we just did not see the version
in the --remote=. above?

> From my understanding, we can only add a single
> version=X field in the advertisement, but IIUC we can extend this fairly
> easily? Perhaps we can have "version=X" to mean the preferred version,
> and then a repeatable "acceptable_version=Y" field or similar?

Just re-use "version X", separated by colons as above.

> > From a maintenance perspective, do we want to keep
> > this part of the code central, as it ties protocol (as proxied
> > by service name) to the max version number?
> > I would think that we'd rather have the decision local to the
> > code, i.e. builtin/fetch would need to tell protocol.c that it
> > can do (0,1,2) and builtin/push can do (0,1), and then the
> > networking layers of code would figure out by the input
> > from the caller and the input from the user (configured
> > protocol.version) what is the best to go forward from
> > then on.
>
> I like having it centralized, because enforcing this in git_connect()
> and discover_refs() catches all the outgoing version advertisements, but
> there's lots of code paths that lead to those two functions that would
> all have to have the acceptable version numbers plumbed through.

Makes sense.

> I suppose we could also have a registry of services to version numbers,
> but I tend to dislike non-local sources of data. But if the list likes
> that approach better, I'll be happy to implement it.

> > But I guess having the central place here is not to
> > bad either. How will it cope with the desire of protocol v2
> > to have only one end point (c.f. serve.{c,h} via builtin/serve
> > as "git serve") ?
>
> I'm not sure about this. In my series to add a v2 archive command, I
> added support for a new endpoint for proto v2 and I don't recall seeing
> any complaints, but that is still open for review.

Ah I guess new end points would imply that you can speak at least
a given version N.

> I suppose if we are strict about serving from a single endpoint, the
> version registry makes more sense, and individual operations can declare
> acceptable version numbers before calling any network code?

Ah yeah, that makes sense!

Thanks for your explanations and prodding,
Stefan

Re: [PATCH 1/1] protocol: limit max protocol version per service

[Josh Steadmon]

On 2018.10.03 15:47, Stefan Beller wrote:
> On Wed, Oct 3, 2018 at 2:34 PM Josh Steadmon <steadmon@google.com> wrote:
> >
> > Is there a method or design for advertising multiple acceptable versions
> > from the client?
> 
> I think the client can send multiple versions, looking through protocol.c
> (and not the documentation as I should for this:)
> 
>   /*
>    * Determine which protocol version the client has requested.  Since
>    * multiple 'version' keys can be sent by the client, indicating that
>    * the client is okay to speak any of them, select the greatest version
>    * that the client has requested.  This is due to the assumption that
>    * the most recent protocol version will be the most state-of-the-art.
>    */
>     ...
>     const char *git_protocol = getenv(GIT_PROTOCOL_ENVIRONMENT);
>     string_list_split(&list, git_protocol, ':', -1);
>     ...
>     for_each_string_list_item(item, &list) {
>         ...
>         if (skip_prefix(item->string, "version=", &value))
>             ...
> 
> in determine_protocol_version_server which already had the client
> speak to it, so I think at least the server can deal with multiple versions.
> 
> But given that we transport the version in env variables, we'd
> need to be extra careful if we just did not see the version
> in the --remote=. above?

Sorry, I'm not sure I understand this. What about env variables requires
caution?


> > From my understanding, we can only add a single
> > version=X field in the advertisement, but IIUC we can extend this fairly
> > easily? Perhaps we can have "version=X" to mean the preferred version,
> > and then a repeatable "acceptable_version=Y" field or similar?
> 
> Just re-use "version X", separated by colons as above.
> 
> > > From a maintenance perspective, do we want to keep
> > > this part of the code central, as it ties protocol (as proxied
> > > by service name) to the max version number?
> > > I would think that we'd rather have the decision local to the
> > > code, i.e. builtin/fetch would need to tell protocol.c that it
> > > can do (0,1,2) and builtin/push can do (0,1), and then the
> > > networking layers of code would figure out by the input
> > > from the caller and the input from the user (configured
> > > protocol.version) what is the best to go forward from
> > > then on.
> >
> > I like having it centralized, because enforcing this in git_connect()
> > and discover_refs() catches all the outgoing version advertisements, but
> > there's lots of code paths that lead to those two functions that would
> > all have to have the acceptable version numbers plumbed through.
> 
> Makes sense.
> 
> > I suppose we could also have a registry of services to version numbers,
> > but I tend to dislike non-local sources of data. But if the list likes
> > that approach better, I'll be happy to implement it.
> 
> > > But I guess having the central place here is not to
> > > bad either. How will it cope with the desire of protocol v2
> > > to have only one end point (c.f. serve.{c,h} via builtin/serve
> > > as "git serve") ?
> >
> > I'm not sure about this. In my series to add a v2 archive command, I
> > added support for a new endpoint for proto v2 and I don't recall seeing
> > any complaints, but that is still open for review.
> 
> Ah I guess new end points would imply that you can speak at least
> a given version N.
> 
> > I suppose if we are strict about serving from a single endpoint, the
> > version registry makes more sense, and individual operations can declare
> > acceptable version numbers before calling any network code?
> 
> Ah yeah, that makes sense!

Thinking out loud here. Please let me know if I say something stupid :)

So we'll have (up to) three pieces of version information we'll care
about for version negotiation:

1. (maybe) a client-side protocol.version config entry
2. a list of acceptable proto versions for the currently running
   operation on the client
3. a list of acceptable proto versions for the server endpoint that
   handles the request

According to the doc on protocol.version: "If set, clients will attempt
to communicate with a server using the specified protocol version. If
unset, no attempt will be made by the client to communicate using a
particular protocol version, this results in protocol version 0 being
used."

So, if protocol.version is not set, or set to 0, the client should not
attempt any sort of version negotiation. Otherwise, the client prefers a
particular version, but we don't guarantee that they will actually use
that version after the (unspecified) version negotiation procedure.

If protocol.version is set to something other than 0, we construct a
list of acceptable versions for the given operation. If the
protocol.version entry is present in that list, we move it to the front
of the list to note that it is the preferred version. We send all of
these, in order, in the request.

When the server endpoint begins to handle a request, it first constructs
a list of acceptable versions. If the client specifies a list of
versions, we check them one-by-one to see if they are acceptable. If we
find a match, we use that version. If we don't find any matches or if
the client did not send a version list, we default to v0.

Seem reasonable?


Thanks,
Josh

Re: [PATCH 1/1] protocol: limit max protocol version per service

[Stefan Beller]

> > But given that we transport the version in env variables, we'd
> > need to be extra careful if we just did not see the version
> > in the --remote=. above?
>
> Sorry, I'm not sure I understand this. What about env variables requires
> caution?

By locally transporting the version via env variables means the absence of
"version X" lines in the GIT_TRACE output is not a clear indication
of version 0, I would think. It is a strong indicator, but now we'd have to dig
and see if the env variables were set outside?


> >
> > > I suppose if we are strict about serving from a single endpoint, the
> > > version registry makes more sense, and individual operations can declare
> > > acceptable version numbers before calling any network code?
> >
> > Ah yeah, that makes sense!
>
> Thinking out loud here. Please let me know if I say something stupid :)
>
> So we'll have (up to) three pieces of version information we'll care
> about for version negotiation:
>
> 1. (maybe) a client-side protocol.version config entry

(and in case we don't, we have it implicit ly hardcoded, as
we have to choose the default for users that don't care themselves about
this setting)

> 2. a list of acceptable proto versions for the currently running
>    operation on the client
> 3. a list of acceptable proto versions for the server endpoint that
>    handles the request

Yes that matches my understanding. The issue is between (1) and (2)
as (1) is in a generic config, whereas (2) is specific to the command,
such that it may differ. And as a user you may want to express things
like: "use the highest version", which is done by setting (1) to "version 2"
despite (2) not having support of all commands for v2.

> According to the doc on protocol.version: "If set, clients will attempt
> to communicate with a server using the specified protocol version. If
> unset, no attempt will be made by the client to communicate using a
> particular protocol version, this results in protocol version 0 being
> used."
>
> So, if protocol.version is not set, or set to 0, the client should not
> attempt any sort of version negotiation.

Yes, as version 0 is unaware of versions, i.e. there are old installations
out there where all the versioning code is not there, so in case of an
old client the new server *must* speak v0 to be able to communicate
(and vice versa).


> Otherwise, the client prefers a
> particular version, but we don't guarantee that they will actually use
> that version after the (unspecified) version negotiation procedure.
>
> If protocol.version is set to something other than 0, we construct a
> list of acceptable versions for the given operation. If the
> protocol.version entry is present in that list, we move it to the front
> of the list to note that it is the preferred version. We send all of
> these, in order, in the request.
>
> When the server endpoint begins to handle a request, it first constructs
> a list of acceptable versions. If the client specifies a list of
> versions, we check them one-by-one to see if they are acceptable. If we
> find a match, we use that version. If we don't find any matches or if
> the client did not send a version list, we default to v0.
>
> Seem reasonable?

This sounds super reasonable!

Thanks
Stefan

Re: [PATCH 1/1] protocol: limit max protocol version per service

[Josh Steadmon]

On 2018.10.05 12:25, Stefan Beller wrote:
> > > > I suppose if we are strict about serving from a single endpoint, the
> > > > version registry makes more sense, and individual operations can declare
> > > > acceptable version numbers before calling any network code?
> > >
> > > Ah yeah, that makes sense!
> >
> > Thinking out loud here. Please let me know if I say something stupid :)
> >
> > So we'll have (up to) three pieces of version information we'll care
> > about for version negotiation:
> >
> > 1. (maybe) a client-side protocol.version config entry
> 
> (and in case we don't, we have it implicit ly hardcoded, as
> we have to choose the default for users that don't care themselves about
> this setting)
> 
> > 2. a list of acceptable proto versions for the currently running
> >    operation on the client
> > 3. a list of acceptable proto versions for the server endpoint that
> >    handles the request
> 
> Yes that matches my understanding. The issue is between (1) and (2)
> as (1) is in a generic config, whereas (2) is specific to the command,
> such that it may differ. And as a user you may want to express things
> like: "use the highest version", which is done by setting (1) to "version 2"
> despite (2) not having support of all commands for v2.
> 
> > According to the doc on protocol.version: "If set, clients will attempt
> > to communicate with a server using the specified protocol version. If
> > unset, no attempt will be made by the client to communicate using a
> > particular protocol version, this results in protocol version 0 being
> > used."
> >
> > So, if protocol.version is not set, or set to 0, the client should not
> > attempt any sort of version negotiation.
> 
> Yes, as version 0 is unaware of versions, i.e. there are old installations
> out there where all the versioning code is not there, so in case of an
> old client the new server *must* speak v0 to be able to communicate
> (and vice versa).
> 
> 
> > Otherwise, the client prefers a
> > particular version, but we don't guarantee that they will actually use
> > that version after the (unspecified) version negotiation procedure.
> >
> > If protocol.version is set to something other than 0, we construct a
> > list of acceptable versions for the given operation. If the
> > protocol.version entry is present in that list, we move it to the front
> > of the list to note that it is the preferred version. We send all of
> > these, in order, in the request.
> >
> > When the server endpoint begins to handle a request, it first constructs
> > a list of acceptable versions. If the client specifies a list of
> > versions, we check them one-by-one to see if they are acceptable. If we
> > find a match, we use that version. If we don't find any matches or if
> > the client did not send a version list, we default to v0.
> >
> > Seem reasonable?
> 
> This sounds super reasonable!

So this runs into problems with remote-curl (and possibly other remote
helpers):

builtin/push.c can declare whatever allowed versions it wants, but those
are not carried over when remote-curl is started to actually talk to the
remote. What's worse, remote-curl starts its HTTP connection before it
knows what command it's actually acting as a helper for.

For now, I'm going to try adding an --allowed_versions flag for the
remote helpers, but if anyone has a better idea, let me know.


Thanks,
Josh

Re: [PATCH 1/1] protocol: limit max protocol version per service

[Jonathan Nieder]

Hi,

Josh Steadmon wrote:

> So this runs into problems with remote-curl (and possibly other remote
> helpers):
>
> builtin/push.c can declare whatever allowed versions it wants, but those
> are not carried over when remote-curl is started to actually talk to the
> remote. What's worse, remote-curl starts its HTTP connection before it
> knows what command it's actually acting as a helper for.
>
> For now, I'm going to try adding an --allowed_versions flag for the
> remote helpers, but if anyone has a better idea, let me know.

There are some external remote helpers (see "git help remote-helpers"
for the documented interface), so alas, they can't take new flags.

That said, you can add a new remote helper capability and then
communicate on stdin, or in a pinch you can use the existing "option"
capability.  Remote helpers are also allowed to query "git config" if
they want to (either using the config machinery in config.h or by
running "git config").

Thanks,
Jonathan

Re: [PATCH 1/1] protocol: limit max protocol version per service

[Jonathan Nieder]

Josh Steadmon wrote:

> For now, I'm going to try adding an --allowed_versions flag for the
> remote helpers, but if anyone has a better idea, let me know.

I forgot to mention: the stateless-connect remote helper capability is
still experimental so you're free to change its interface (e.g. to
change the syntax of the stateless-connect command it provides).

Thanks again,
Jonathan

Re: [PATCH 1/1] protocol: limit max protocol version per service

[Josh Steadmon]

On 2018.10.12 16:32, Jonathan Nieder wrote:
> Josh Steadmon wrote:
> 
> > For now, I'm going to try adding an --allowed_versions flag for the
> > remote helpers, but if anyone has a better idea, let me know.
> 
> I forgot to mention: the stateless-connect remote helper capability is
> still experimental so you're free to change its interface (e.g. to
> change the syntax of the stateless-connect command it provides).

For v2 of this patch, I ended up using GIT_PROTOCOL to pass the version
string to the remote helpers.

Re: [PATCH 1/1] protocol: limit max protocol version per service

[Jonathan Nieder]

Josh Steadmon wrote:
> On 2018.10.12 16:32, Jonathan Nieder wrote:
>> Josh Steadmon wrote:

>>> For now, I'm going to try adding an --allowed_versions flag for the
>>> remote helpers, but if anyone has a better idea, let me know.
>>
>> I forgot to mention: the stateless-connect remote helper capability is
>> still experimental so you're free to change its interface (e.g. to
>> change the syntax of the stateless-connect command it provides).
>
> For v2 of this patch, I ended up using GIT_PROTOCOL to pass the version
> string to the remote helpers.

Makes sense.  Thanks. :)

Re: [PATCH 1/1] protocol: limit max protocol version per service

[Junio C Hamano]

Jonathan Nieder <jrnieder@gmail.com> writes:

> Josh Steadmon wrote:
>> On 2018.10.12 16:32, Jonathan Nieder wrote:
>>> Josh Steadmon wrote:
>
>>>> For now, I'm going to try adding an --allowed_versions flag for the
>>>> remote helpers, but if anyone has a better idea, let me know.
>>>
>>> I forgot to mention: the stateless-connect remote helper capability is
>>> still experimental so you're free to change its interface (e.g. to
>>> change the syntax of the stateless-connect command it provides).
>>
>> For v2 of this patch, I ended up using GIT_PROTOCOL to pass the version
>> string to the remote helpers.
>
> Makes sense.  Thanks. :)

Yeah, it does.

[PATCH v2 0/1] Advertise multiple supported proto versions

[steadmon]

From: Josh Steadmon <steadmon@google.com>

This is an alternate approach to the previous series. We add a registry
of supported wire protocol versions that individual commands can use to
declare supported versions before contacting a server. The client will
then advertise all supported versions, while the server will choose the
first recognized version from the advertised list.

Compared to the previous series, this approach is more convenient for
protocol_v2, which is intended to work on a single server endpoint.
However, it has the drawback that every command that acts as a client
must register its supported versions; it is not always obvious which (if
any) network operations a given command will perform.

Thank you to Stefan for his review of the previous series and for
helping me think through the requirements for this new approach.

Josh Steadmon (1):
  protocol: advertise multiple supported versions

 builtin/archive.c      |   3 ++
 builtin/clone.c        |   4 ++
 builtin/fetch-pack.c   |   4 ++
 builtin/fetch.c        |   5 ++
 builtin/ls-remote.c    |   5 ++
 builtin/pull.c         |   5 ++
 builtin/push.c         |   4 ++
 builtin/send-pack.c    |   3 ++
 connect.c              |  47 ++++++++---------
 protocol.c             | 115 ++++++++++++++++++++++++++++++++++++++---
 protocol.h             |  17 ++++++
 remote-curl.c          |  28 ++++++----
 t/t5570-git-daemon.sh  |   2 +-
 t/t5700-protocol-v1.sh |   8 +--
 t/t5702-protocol-v2.sh |  16 +++---
 transport-helper.c     |   6 +++
 16 files changed, 217 insertions(+), 55 deletions(-)

-- 
2.19.0.605.g01d371f741-goog

[PATCH v2 1/1] protocol: advertise multiple supported versions

[steadmon]

From: Josh Steadmon <steadmon@google.com>

Currently the client advertises that it supports the wire protocol
version set in the protocol.version config. However, not all services
support the same set of protocol versions. When connecting to
git-receive-pack, the client automatically downgrades to v0 if
config.protocol is set to v2, but this check is not performed for other
services.

This patch creates a protocol version registry. Individual commands
register all the protocol versions they support prior to communicating
with a server. Versions should be listed in preference order; the
version specified in protocol.version will automatically be moved to the
front of the registry.

The protocol version registry is passed to remote helpers via the
GIT_PROTOCOL environment variable.

Clients now advertise the full list of registered versions. Servers
select the first recognized version from this advertisement.

Signed-off-by: Josh Steadmon <steadmon@google.com>
---
 builtin/archive.c      |   3 ++
 builtin/clone.c        |   4 ++
 builtin/fetch-pack.c   |   4 ++
 builtin/fetch.c        |   5 ++
 builtin/ls-remote.c    |   5 ++
 builtin/pull.c         |   5 ++
 builtin/push.c         |   4 ++
 builtin/send-pack.c    |   3 ++
 connect.c              |  47 ++++++++---------
 protocol.c             | 115 ++++++++++++++++++++++++++++++++++++++---
 protocol.h             |  17 ++++++
 remote-curl.c          |  28 ++++++----
 t/t5570-git-daemon.sh  |   2 +-
 t/t5700-protocol-v1.sh |   8 +--
 t/t5702-protocol-v2.sh |  16 +++---
 transport-helper.c     |   6 +++
 16 files changed, 217 insertions(+), 55 deletions(-)

diff --git a/builtin/archive.c b/builtin/archive.c
index e74f675390..8adb9f381b 100644
--- a/builtin/archive.c
+++ b/builtin/archive.c
@@ -8,6 +8,7 @@
 #include "transport.h"
 #include "parse-options.h"
 #include "pkt-line.h"
+#include "protocol.h"
 #include "sideband.h"
 
 static void create_output_file(const char *output_file)
@@ -94,6 +95,8 @@ int cmd_archive(int argc, const char **argv, const char *prefix)
 		OPT_END()
 	};
 
+	register_allowed_protocol_version(protocol_v0);
+
 	argc = parse_options(argc, argv, prefix, local_opts, NULL,
 			     PARSE_OPT_KEEP_ALL);
 
diff --git a/builtin/clone.c b/builtin/clone.c
index fd2c3ef090..1651a950b6 100644
--- a/builtin/clone.c
+++ b/builtin/clone.c
@@ -900,6 +900,10 @@ int cmd_clone(int argc, const char **argv, const char *prefix)
 	struct refspec rs = REFSPEC_INIT_FETCH;
 	struct argv_array ref_prefixes = ARGV_ARRAY_INIT;
 
+	register_allowed_protocol_version(protocol_v2);
+	register_allowed_protocol_version(protocol_v1);
+	register_allowed_protocol_version(protocol_v0);
+
 	fetch_if_missing = 0;
 
 	packet_trace_identity("clone");
diff --git a/builtin/fetch-pack.c b/builtin/fetch-pack.c
index 1a1bc63566..cba935e4d3 100644
--- a/builtin/fetch-pack.c
+++ b/builtin/fetch-pack.c
@@ -57,6 +57,10 @@ int cmd_fetch_pack(int argc, const char **argv, const char *prefix)
 
 	fetch_if_missing = 0;
 
+	register_allowed_protocol_version(protocol_v2);
+	register_allowed_protocol_version(protocol_v1);
+	register_allowed_protocol_version(protocol_v0);
+
 	packet_trace_identity("fetch-pack");
 
 	memset(&args, 0, sizeof(args));
diff --git a/builtin/fetch.c b/builtin/fetch.c
index 61bec5d213..2a20cf8bfd 100644
--- a/builtin/fetch.c
+++ b/builtin/fetch.c
@@ -21,6 +21,7 @@
 #include "argv-array.h"
 #include "utf8.h"
 #include "packfile.h"
+#include "protocol.h"
 #include "list-objects-filter-options.h"
 
 static const char * const builtin_fetch_usage[] = {
@@ -1476,6 +1477,10 @@ int cmd_fetch(int argc, const char **argv, const char *prefix)
 	int prune_tags_ok = 1;
 	struct argv_array argv_gc_auto = ARGV_ARRAY_INIT;
 
+	register_allowed_protocol_version(protocol_v2);
+	register_allowed_protocol_version(protocol_v1);
+	register_allowed_protocol_version(protocol_v0);
+
 	packet_trace_identity("fetch");
 
 	fetch_if_missing = 0;
diff --git a/builtin/ls-remote.c b/builtin/ls-remote.c
index 1a25df7ee1..ea685e8bb9 100644
--- a/builtin/ls-remote.c
+++ b/builtin/ls-remote.c
@@ -1,5 +1,6 @@
 #include "builtin.h"
 #include "cache.h"
+#include "protocol.h"
 #include "transport.h"
 #include "ref-filter.h"
 #include "remote.h"
@@ -80,6 +81,10 @@ int cmd_ls_remote(int argc, const char **argv, const char *prefix)
 
 	memset(&ref_array, 0, sizeof(ref_array));
 
+	register_allowed_protocol_version(protocol_v2);
+	register_allowed_protocol_version(protocol_v1);
+	register_allowed_protocol_version(protocol_v0);
+
 	argc = parse_options(argc, argv, prefix, options, ls_remote_usage,
 			     PARSE_OPT_STOP_AT_NON_OPTION);
 	dest = argv[0];
diff --git a/builtin/pull.c b/builtin/pull.c
index 681c127a07..cb64146834 100644
--- a/builtin/pull.c
+++ b/builtin/pull.c
@@ -9,6 +9,7 @@
 #include "config.h"
 #include "builtin.h"
 #include "parse-options.h"
+#include "protocol.h"
 #include "exec-cmd.h"
 #include "run-command.h"
 #include "sha1-array.h"
@@ -849,6 +850,10 @@ int cmd_pull(int argc, const char **argv, const char *prefix)
 	struct object_id rebase_fork_point;
 	int autostash;
 
+	register_allowed_protocol_version(protocol_v2);
+	register_allowed_protocol_version(protocol_v1);
+	register_allowed_protocol_version(protocol_v0);
+
 	if (!getenv("GIT_REFLOG_ACTION"))
 		set_reflog_message(argc, argv);
 
diff --git a/builtin/push.c b/builtin/push.c
index d09a42062c..10d8abe829 100644
--- a/builtin/push.c
+++ b/builtin/push.c
@@ -10,6 +10,7 @@
 #include "remote.h"
 #include "transport.h"
 #include "parse-options.h"
+#include "protocol.h"
 #include "submodule.h"
 #include "submodule-config.h"
 #include "send-pack.h"
@@ -587,6 +588,9 @@ int cmd_push(int argc, const char **argv, const char *prefix)
 		OPT_END()
 	};
 
+	register_allowed_protocol_version(protocol_v1);
+	register_allowed_protocol_version(protocol_v0);
+
 	packet_trace_identity("push");
 	git_config(git_push_config, &flags);
 	argc = parse_options(argc, argv, prefix, options, push_usage, 0);
diff --git a/builtin/send-pack.c b/builtin/send-pack.c
index 8e3c7490f7..f48bd1306b 100644
--- a/builtin/send-pack.c
+++ b/builtin/send-pack.c
@@ -184,6 +184,9 @@ int cmd_send_pack(int argc, const char **argv, const char *prefix)
 		OPT_END()
 	};
 
+	register_allowed_protocol_version(protocol_v1);
+	register_allowed_protocol_version(protocol_v0);
+
 	git_config(send_pack_config, NULL);
 	argc = parse_options(argc, argv, prefix, options, send_pack_usage, 0);
 	if (argc > 0) {
diff --git a/connect.c b/connect.c
index 94547e5056..a170ba1e90 100644
--- a/connect.c
+++ b/connect.c
@@ -1046,7 +1046,7 @@ static enum ssh_variant determine_ssh_variant(const char *ssh_command,
  */
 static struct child_process *git_connect_git(int fd[2], char *hostandport,
 					     const char *path, const char *prog,
-					     enum protocol_version version,
+					     const struct strbuf *version_advert,
 					     int flags)
 {
 	struct child_process *conn;
@@ -1085,10 +1085,9 @@ static struct child_process *git_connect_git(int fd[2], char *hostandport,
 		    target_host, 0);
 
 	/* If using a new version put that stuff here after a second null byte */
-	if (version > 0) {
+	if (strcmp(version_advert->buf, "version=0")) {
 		strbuf_addch(&request, '\0');
-		strbuf_addf(&request, "version=%d%c",
-			    version, '\0');
+		strbuf_addf(&request, "%s%c", version_advert->buf, '\0');
 	}
 
 	packet_write(fd[1], request.buf, request.len);
@@ -1104,14 +1103,14 @@ static struct child_process *git_connect_git(int fd[2], char *hostandport,
  */
 static void push_ssh_options(struct argv_array *args, struct argv_array *env,
 			     enum ssh_variant variant, const char *port,
-			     enum protocol_version version, int flags)
+			     const struct strbuf *version_advert, int flags)
 {
 	if (variant == VARIANT_SSH &&
-	    version > 0) {
+	    strcmp(version_advert->buf, "version=0")) {
 		argv_array_push(args, "-o");
 		argv_array_push(args, "SendEnv=" GIT_PROTOCOL_ENVIRONMENT);
-		argv_array_pushf(env, GIT_PROTOCOL_ENVIRONMENT "=version=%d",
-				 version);
+		argv_array_pushf(env, GIT_PROTOCOL_ENVIRONMENT "=%s",
+				 version_advert->buf);
 	}
 
 	if (flags & CONNECT_IPV4) {
@@ -1164,7 +1163,7 @@ static void push_ssh_options(struct argv_array *args, struct argv_array *env,
 
 /* Prepare a child_process for use by Git's SSH-tunneled transport. */
 static void fill_ssh_args(struct child_process *conn, const char *ssh_host,
-			  const char *port, enum protocol_version version,
+			  const char *port, const struct strbuf *version_advert,
 			  int flags)
 {
 	const char *ssh;
@@ -1198,15 +1197,16 @@ static void fill_ssh_args(struct child_process *conn, const char *ssh_host,
 
 		argv_array_push(&detect.args, ssh);
 		argv_array_push(&detect.args, "-G");
-		push_ssh_options(&detect.args, &detect.env_array,
-				 VARIANT_SSH, port, version, flags);
+		push_ssh_options(&detect.args, &detect.env_array, VARIANT_SSH,
+				 port, version_advert, flags);
 		argv_array_push(&detect.args, ssh_host);
 
 		variant = run_command(&detect) ? VARIANT_SIMPLE : VARIANT_SSH;
 	}
 
 	argv_array_push(&conn->args, ssh);
-	push_ssh_options(&conn->args, &conn->env_array, variant, port, version, flags);
+	push_ssh_options(&conn->args, &conn->env_array, variant, port,
+			 version_advert, flags);
 	argv_array_push(&conn->args, ssh_host);
 }
 
@@ -1226,16 +1226,10 @@ struct child_process *git_connect(int fd[2], const char *url,
 {
 	char *hostandport, *path;
 	struct child_process *conn;
+	struct strbuf version_advert = STRBUF_INIT;
 	enum protocol protocol;
-	enum protocol_version version = get_protocol_version_config();
 
-	/*
-	 * NEEDSWORK: If we are trying to use protocol v2 and we are planning
-	 * to perform a push, then fallback to v0 since the client doesn't know
-	 * how to push yet using v2.
-	 */
-	if (version == protocol_v2 && !strcmp("git-receive-pack", prog))
-		version = protocol_v0;
+	get_client_protocol_version_advertisement(&version_advert);
 
 	/* Without this we cannot rely on waitpid() to tell
 	 * what happened to our children.
@@ -1250,7 +1244,8 @@ struct child_process *git_connect(int fd[2], const char *url,
 		printf("Diag: path=%s\n", path ? path : "NULL");
 		conn = NULL;
 	} else if (protocol == PROTO_GIT) {
-		conn = git_connect_git(fd, hostandport, path, prog, version, flags);
+		conn = git_connect_git(fd, hostandport, path, prog,
+				       &version_advert, flags);
 	} else {
 		struct strbuf cmd = STRBUF_INIT;
 		const char *const *var;
@@ -1293,12 +1288,14 @@ struct child_process *git_connect(int fd[2], const char *url,
 				strbuf_release(&cmd);
 				return NULL;
 			}
-			fill_ssh_args(conn, ssh_host, port, version, flags);
+			fill_ssh_args(conn, ssh_host, port, &version_advert,
+				      flags);
 		} else {
 			transport_check_allowed("file");
-			if (version > 0) {
-				argv_array_pushf(&conn->env_array, GIT_PROTOCOL_ENVIRONMENT "=version=%d",
-						 version);
+			if (strcmp(version_advert.buf, "version=0")) {
+				argv_array_pushf(&conn->env_array,
+						 GIT_PROTOCOL_ENVIRONMENT "=%s",
+						 version_advert.buf);
 			}
 		}
 		argv_array_push(&conn->args, cmd.buf);
diff --git a/protocol.c b/protocol.c
index 5e636785d1..f788269c47 100644
--- a/protocol.c
+++ b/protocol.c
@@ -2,18 +2,43 @@
 #include "config.h"
 #include "protocol.h"
 
+static enum protocol_version *allowed_versions;
+static int nr_allowed_versions;
+static int alloc_allowed_versions;
+static int have_advertised_versions_already = 0;
+
+static const char protocol_v0_string[] = "0";
+static const char protocol_v1_string[] = "1";
+static const char protocol_v2_string[] = "2";
+
 static enum protocol_version parse_protocol_version(const char *value)
 {
-	if (!strcmp(value, "0"))
+	if (!strcmp(value, protocol_v0_string))
 		return protocol_v0;
-	else if (!strcmp(value, "1"))
+	else if (!strcmp(value, protocol_v1_string))
 		return protocol_v1;
-	else if (!strcmp(value, "2"))
+	else if (!strcmp(value, protocol_v2_string))
 		return protocol_v2;
 	else
 		return protocol_unknown_version;
 }
 
+/* Return the text representation of a wire protocol version. */
+static const char *format_protocol_version(enum protocol_version version)
+{
+	switch (version) {
+	case protocol_v0:
+		return protocol_v0_string;
+	case protocol_v1:
+		return protocol_v1_string;
+	case protocol_v2:
+		return protocol_v2_string;
+	case protocol_unknown_version:
+		die(_("Unrecognized protocol version"));
+	}
+	die(_("Unrecognized protocol_version"));
+}
+
 enum protocol_version get_protocol_version_config(void)
 {
 	const char *value;
@@ -30,6 +55,79 @@ enum protocol_version get_protocol_version_config(void)
 	return protocol_v0;
 }
 
+void register_allowed_protocol_version(enum protocol_version version)
+{
+	if (have_advertised_versions_already)
+		error(_("attempting to register an allowed protocol version after advertisement"));
+
+	ALLOC_GROW(allowed_versions, nr_allowed_versions + 1,
+		   alloc_allowed_versions);
+	allowed_versions[nr_allowed_versions++] = version;
+}
+
+void register_allowed_protocol_versions_from_env(void)
+{
+	const char *git_protocol = getenv(GIT_PROTOCOL_ENVIRONMENT);
+	const char *version_str;
+	struct string_list version_list = STRING_LIST_INIT_DUP;
+	struct string_list_item *version;
+
+	if (!git_protocol)
+		return;
+
+	string_list_split(&version_list, git_protocol, ':', -1);
+	for_each_string_list_item(version, &version_list) {
+		if (skip_prefix(version->string, "version=", &version_str))
+			register_allowed_protocol_version(
+				parse_protocol_version(version_str));
+	}
+	string_list_clear(&version_list, 0);
+}
+
+void get_client_protocol_version_advertisement(struct strbuf *advert)
+{
+	int tmp_nr = nr_allowed_versions;
+	enum protocol_version *tmp_allowed_versions, config_version;
+	strbuf_reset(advert);
+
+	have_advertised_versions_already = 1;
+
+	config_version = get_protocol_version_config();
+	if (config_version == protocol_v0) {
+		strbuf_addstr(advert, "version=0");
+		return;
+	}
+
+	if (tmp_nr > 0) {
+		ALLOC_ARRAY(tmp_allowed_versions, tmp_nr);
+		copy_array(tmp_allowed_versions, allowed_versions, tmp_nr,
+			   sizeof(enum protocol_version));
+	} else {
+		ALLOC_ARRAY(tmp_allowed_versions, 1);
+		tmp_nr = 1;
+		tmp_allowed_versions[0] = config_version;
+	}
+
+	if (tmp_allowed_versions[0] != config_version)
+		for (int i = 1; i < nr_allowed_versions; i++)
+			if (tmp_allowed_versions[i] == config_version) {
+				enum protocol_version swap =
+					tmp_allowed_versions[0];
+				tmp_allowed_versions[0] =
+					tmp_allowed_versions[i];
+				tmp_allowed_versions[i] = swap;
+			}
+
+	strbuf_addf(advert, "version=%s",
+		    format_protocol_version(tmp_allowed_versions[0]));
+	for (int i = 1; i < tmp_nr; i++)
+		strbuf_addf(advert, ":version=%s",
+			    format_protocol_version(tmp_allowed_versions[i]));
+
+}
+
 enum protocol_version determine_protocol_version_server(void)
 {
 	const char *git_protocol = getenv(GIT_PROTOCOL_ENVIRONMENT);
@@ -38,9 +136,10 @@ enum protocol_version determine_protocol_version_server(void)
 	/*
 	 * Determine which protocol version the client has requested.  Since
 	 * multiple 'version' keys can be sent by the client, indicating that
-	 * the client is okay to speak any of them, select the greatest version
-	 * that the client has requested.  This is due to the assumption that
-	 * the most recent protocol version will be the most state-of-the-art.
+	 * the client is okay to speak any of them, select the first
+	 * recognizable version that the client has requested.  This is due to
+	 * the assumption that the protocol versions will be listed in
+	 * preference order.
 	 */
 	if (git_protocol) {
 		struct string_list list = STRING_LIST_INIT_DUP;
@@ -53,8 +152,10 @@ enum protocol_version determine_protocol_version_server(void)
 
 			if (skip_prefix(item->string, "version=", &value)) {
 				v = parse_protocol_version(value);
-				if (v > version)
+				if (v != protocol_unknown_version) {
 					version = v;
+					break;
+				}
 			}
 		}
 
diff --git a/protocol.h b/protocol.h
index 2ad35e433c..b67b2259de 100644
--- a/protocol.h
+++ b/protocol.h
@@ -16,6 +16,23 @@ enum protocol_version {
  */
 extern enum protocol_version get_protocol_version_config(void);
 
+/*
+ * Register an allowable protocol version for a given operation. Registration
+ * must occur before attempting to advertise a version to a server process.
+ */
+extern void register_allowed_protocol_version(enum protocol_version version);
+
+/*
+ * Register allowable protocol versions from the GIT_PROTOCOL environment var.
+ */
+extern void register_allowed_protocol_versions_from_env(void);
+
+/*
+ * Fill a strbuf with a version advertisement string suitable for use in the
+ * GIT_PROTOCOL environment variable or similar version negotiation field.
+ */
+extern void get_client_protocol_version_advertisement(struct strbuf *advert);
+
 /*
  * Used by a server to determine which protocol version should be used based on
  * a client's request, communicated via the 'GIT_PROTOCOL' environment variable
diff --git a/remote-curl.c b/remote-curl.c
index fb28309e85..537f8c7aac 100644
--- a/remote-curl.c
+++ b/remote-curl.c
@@ -330,6 +330,20 @@ static int get_protocol_http_header(enum protocol_version version,
 	return 0;
 }
 
+static int get_client_protocol_http_header(const struct strbuf *version_advert,
+					   struct strbuf *header)
+{
+	if (version_advert->len > 0 &&
+	    strcmp(version_advert->buf, "version=0")) {
+		strbuf_addf(header, GIT_PROTOCOL_HEADER ": %s",
+			    version_advert->buf);
+
+		return 1;
+	}
+
+	return 0;
+}
+
 static struct discovery *discover_refs(const char *service, int for_push)
 {
 	struct strbuf exp = STRBUF_INIT;
@@ -339,11 +353,11 @@ static struct discovery *discover_refs(const char *service, int for_push)
 	struct strbuf refs_url = STRBUF_INIT;
 	struct strbuf effective_url = STRBUF_INIT;
 	struct strbuf protocol_header = STRBUF_INIT;
+	struct strbuf version_advert = STRBUF_INIT;
 	struct string_list extra_headers = STRING_LIST_INIT_DUP;
 	struct discovery *last = last_discovery;
 	int http_ret, maybe_smart = 0;
 	struct http_get_options http_options;
-	enum protocol_version version = get_protocol_version_config();
 
 	if (last && !strcmp(service, last->service))
 		return last;
@@ -360,16 +374,10 @@ static struct discovery *discover_refs(const char *service, int for_push)
 		strbuf_addf(&refs_url, "service=%s", service);
 	}
 
-	/*
-	 * NEEDSWORK: If we are trying to use protocol v2 and we are planning
-	 * to perform a push, then fallback to v0 since the client doesn't know
-	 * how to push yet using v2.
-	 */
-	if (version == protocol_v2 && !strcmp("git-receive-pack", service))
-		version = protocol_v0;
+	get_client_protocol_version_advertisement(&version_advert);
 
 	/* Add the extra Git-Protocol header */
-	if (get_protocol_http_header(version, &protocol_header))
+	if (get_client_protocol_http_header(&version_advert, &protocol_header))
 		string_list_append(&extra_headers, protocol_header.buf);
 
 	memset(&http_options, 0, sizeof(http_options));
@@ -1327,6 +1335,8 @@ int cmd_main(int argc, const char **argv)
 	struct strbuf buf = STRBUF_INIT;
 	int nongit;
 
+	register_allowed_protocol_versions_from_env();
+
 	setup_git_directory_gently(&nongit);
 	if (argc < 2) {
 		error("remote-curl: usage: git remote-curl <remote> [<url>]");
diff --git a/t/t5570-git-daemon.sh b/t/t5570-git-daemon.sh
index 7466aad111..d528e91630 100755
--- a/t/t5570-git-daemon.sh
+++ b/t/t5570-git-daemon.sh
@@ -186,7 +186,7 @@ test_expect_success 'hostname cannot break out of directory' '
 test_expect_success 'daemon log records all attributes' '
 	cat >expect <<-\EOF &&
 	Extended attribute "host": localhost
-	Extended attribute "protocol": version=1
+	Extended attribute "protocol": version=1:version=2:version=0
 	EOF
 	>daemon.log &&
 	GIT_OVERRIDE_VIRTUAL_HOST=localhost \
diff --git a/t/t5700-protocol-v1.sh b/t/t5700-protocol-v1.sh
index ba86a44eb1..2e56c79233 100755
--- a/t/t5700-protocol-v1.sh
+++ b/t/t5700-protocol-v1.sh
@@ -26,7 +26,7 @@ test_expect_success 'clone with git:// using protocol v1' '
 	test_cmp expect actual &&
 
 	# Client requested to use protocol v1
-	grep "clone> .*\\\0\\\0version=1\\\0$" log &&
+	grep "clone> .*\\\0\\\0version=1.*\\\0$" log &&
 	# Server responded using protocol v1
 	grep "clone< version 1" log
 '
@@ -42,7 +42,7 @@ test_expect_success 'fetch with git:// using protocol v1' '
 	test_cmp expect actual &&
 
 	# Client requested to use protocol v1
-	grep "fetch> .*\\\0\\\0version=1\\\0$" log &&
+	grep "fetch> .*\\\0\\\0version=1.*\\\0$" log &&
 	# Server responded using protocol v1
 	grep "fetch< version 1" log
 '
@@ -56,7 +56,7 @@ test_expect_success 'pull with git:// using protocol v1' '
 	test_cmp expect actual &&
 
 	# Client requested to use protocol v1
-	grep "fetch> .*\\\0\\\0version=1\\\0$" log &&
+	grep "fetch> .*\\\0\\\0version=1.*\\\0$" log &&
 	# Server responded using protocol v1
 	grep "fetch< version 1" log
 '
@@ -74,7 +74,7 @@ test_expect_success 'push with git:// using protocol v1' '
 	test_cmp expect actual &&
 
 	# Client requested to use protocol v1
-	grep "push> .*\\\0\\\0version=1\\\0$" log &&
+	grep "push> .*\\\0\\\0version=1.*\\\0$" log &&
 	# Server responded using protocol v1
 	grep "push< version 1" log
 '
diff --git a/t/t5702-protocol-v2.sh b/t/t5702-protocol-v2.sh
index 3beeed4546..78c17c25e4 100755
--- a/t/t5702-protocol-v2.sh
+++ b/t/t5702-protocol-v2.sh
@@ -24,7 +24,7 @@ test_expect_success 'list refs with git:// using protocol v2' '
 		ls-remote --symref "$GIT_DAEMON_URL/parent" >actual &&
 
 	# Client requested to use protocol v2
-	grep "git> .*\\\0\\\0version=2\\\0$" log &&
+	grep "git> .*\\\0\\\0version=2.*\\\0$" log &&
 	# Server responded using protocol v2
 	grep "git< version 2" log &&
 
@@ -56,7 +56,7 @@ test_expect_success 'clone with git:// using protocol v2' '
 	test_cmp expect actual &&
 
 	# Client requested to use protocol v2
-	grep "clone> .*\\\0\\\0version=2\\\0$" log &&
+	grep "clone> .*\\\0\\\0version=2.*\\\0$" log &&
 	# Server responded using protocol v2
 	grep "clone< version 2" log
 '
@@ -74,7 +74,7 @@ test_expect_success 'fetch with git:// using protocol v2' '
 	test_cmp expect actual &&
 
 	# Client requested to use protocol v2
-	grep "fetch> .*\\\0\\\0version=2\\\0$" log &&
+	grep "fetch> .*\\\0\\\0version=2.*\\\0$" log &&
 	# Server responded using protocol v2
 	grep "fetch< version 2" log
 '
@@ -90,7 +90,7 @@ test_expect_success 'pull with git:// using protocol v2' '
 	test_cmp expect actual &&
 
 	# Client requested to use protocol v2
-	grep "fetch> .*\\\0\\\0version=2\\\0$" log &&
+	grep "fetch> .*\\\0\\\0version=2.*\\\0$" log &&
 	# Server responded using protocol v2
 	grep "fetch< version 2" log
 '
@@ -476,7 +476,7 @@ test_expect_success 'push with http:// and a config of v2 does not request v2' '
 	test_when_finished "rm -f log" &&
 	# Till v2 for push is designed, make sure that if a client has
 	# protocol.version configured to use v2, that the client instead falls
-	# back and uses v0.
+	# back to previous versions.
 
 	test_commit -C http_child three &&
 
@@ -489,10 +489,8 @@ test_expect_success 'push with http:// and a config of v2 does not request v2' '
 	git -C "$HTTPD_DOCUMENT_ROOT_PATH/http_parent" log -1 --format=%s client_branch >expect &&
 	test_cmp expect actual &&
 
-	# Client didnt request to use protocol v2
-	! grep "Git-Protocol: version=2" log &&
-	# Server didnt respond using protocol v2
-	! grep "git< version 2" log
+	# Server responded with version 1
+	grep "git< version 1" log
 '
 
 
diff --git a/transport-helper.c b/transport-helper.c
index 143ca008c8..ac1937f1e1 100644
--- a/transport-helper.c
+++ b/transport-helper.c
@@ -105,6 +105,7 @@ static struct child_process *get_helper(struct transport *transport)
 {
 	struct helper_data *data = transport->data;
 	struct strbuf buf = STRBUF_INIT;
+	struct strbuf version_advert = STRBUF_INIT;
 	struct child_process *helper;
 	int duped;
 	int code;
@@ -127,6 +128,11 @@ static struct child_process *get_helper(struct transport *transport)
 		argv_array_pushf(&helper->env_array, "%s=%s",
 				 GIT_DIR_ENVIRONMENT, get_git_dir());
 
+	get_client_protocol_version_advertisement(&version_advert);
+	if (version_advert.len > 0)
+		argv_array_pushf(&helper->env_array, "%s=%s",
+				 GIT_PROTOCOL_ENVIRONMENT, version_advert.buf);
+
 	code = start_command(helper);
 	if (code < 0 && errno == ENOENT)
 		die(_("unable to find remote helper for '%s'"), data->name);
-- 
2.19.0.605.g01d371f741-goog

Re: [PATCH v2 1/1] protocol: advertise multiple supported versions

[Stefan Beller]

On Thu, Oct 11, 2018 at 6:02 PM <steadmon@google.com> wrote:
>
> From: Josh Steadmon <steadmon@google.com>
>
> Currently the client advertises that it supports the wire protocol
> version set in the protocol.version config. However, not all services
> support the same set of protocol versions. When connecting to
> git-receive-pack, the client automatically downgrades to v0 if
> config.protocol is set to v2, but this check is not performed for other
> services.
>
> This patch creates a protocol version registry. Individual commands
> register all the protocol versions they support prior to communicating
> with a server. Versions should be listed in preference order; the
> version specified in protocol.version will automatically be moved to the
> front of the registry.
>
> The protocol version registry is passed to remote helpers via the
> GIT_PROTOCOL environment variable.
>
> Clients now advertise the full list of registered versions. Servers
> select the first recognized version from this advertisement.

I like this patch from the users point of view (i.e. inside fetch/push etc),
and I need to through the details in connect/protocol as that seems
to be a lot of new code, but hides the complexity very well.

> +       register_allowed_protocol_version(protocol_v2);
> +       register_allowed_protocol_version(protocol_v1);
> +       register_allowed_protocol_version(protocol_v0);

Would it make sense to have a

    register_allowed_protocol_versions(protocol_v2, protocol_v1,
protocol_v0, NULL);

similar to argv_array_pushl  or would that be overengineered?

> @@ -1085,10 +1085,9 @@ static struct child_process *git_connect_git(int fd[2], char *hostandport,
>                     target_host, 0);
>
>         /* If using a new version put that stuff here after a second null byte */
> -       if (version > 0) {
> +       if (strcmp(version_advert->buf, "version=0")) {
>                 strbuf_addch(&request, '\0');
> -               strbuf_addf(&request, "version=%d%c",
> -                           version, '\0');
> +               strbuf_addf(&request, "%s%c", version_advert->buf, '\0');

It's a bit unfortunate to pass around the string, but reading through
the following
lines seems like it is easiest.


> @@ -1226,16 +1226,10 @@ struct child_process *git_connect(int fd[2], const char *url,
>  {
>         char *hostandport, *path;
>         struct child_process *conn;
> +       struct strbuf version_advert = STRBUF_INIT;
>         enum protocol protocol;
> -       enum protocol_version version = get_protocol_version_config();
>
> -       /*
> -        * NEEDSWORK: If we are trying to use protocol v2 and we are planning
> -        * to perform a push, then fallback to v0 since the client doesn't know
> -        * how to push yet using v2.
> -        */
> -       if (version == protocol_v2 && !strcmp("git-receive-pack", prog))
> -               version = protocol_v0;
> +       get_client_protocol_version_advertisement(&version_advert);

Nice to have this special casing gone!

> diff --git a/protocol.c b/protocol.c
> index 5e636785d1..f788269c47 100644
> +
> +static const char protocol_v0_string[] = "0";
> +static const char protocol_v1_string[] = "1";
> +static const char protocol_v2_string[] = "2";
> +
...
> +/* Return the text representation of a wire protocol version. */
> +static const char *format_protocol_version(enum protocol_version version)
> +{
> +       switch (version) {
> +       case protocol_v0:
> +               return protocol_v0_string;
> +       case protocol_v1:
> +               return protocol_v1_string;
> +       case protocol_v2:
> +               return protocol_v2_string;
> +       case protocol_unknown_version:
> +               die(_("Unrecognized protocol version"));
> +       }
> +       die(_("Unrecognized protocol_version"));
> +}

Up to now we have the textual representation that can easily
be constructed from protocol_version by using e.g.
    sprintf(buf, "%d", version);
which is why I initially thought this could be worded way
shorter, but I guess this is more future proof?


> +
>  enum protocol_version get_protocol_version_config(void)
>  {
>         const char *value;
> @@ -30,6 +55,79 @@ enum protocol_version get_protocol_version_config(void)
>         return protocol_v0;
>  }
>
> +void register_allowed_protocol_version(enum protocol_version version)
> +{
> +       if (have_advertised_versions_already)
> +               error(_("attempting to register an allowed protocol version after advertisement"));

This would be a
    BUG(...)
instead?

Re: [PATCH v2 1/1] protocol: advertise multiple supported versions

[Josh Steadmon]

On 2018.10.12 15:30, Stefan Beller wrote:
> On Thu, Oct 11, 2018 at 6:02 PM <steadmon@google.com> wrote:
> >
> > From: Josh Steadmon <steadmon@google.com>
> >
> > Currently the client advertises that it supports the wire protocol
> > version set in the protocol.version config. However, not all services
> > support the same set of protocol versions. When connecting to
> > git-receive-pack, the client automatically downgrades to v0 if
> > config.protocol is set to v2, but this check is not performed for other
> > services.
> >
> > This patch creates a protocol version registry. Individual commands
> > register all the protocol versions they support prior to communicating
> > with a server. Versions should be listed in preference order; the
> > version specified in protocol.version will automatically be moved to the
> > front of the registry.
> >
> > The protocol version registry is passed to remote helpers via the
> > GIT_PROTOCOL environment variable.
> >
> > Clients now advertise the full list of registered versions. Servers
> > select the first recognized version from this advertisement.
> 
> I like this patch from the users point of view (i.e. inside fetch/push etc),
> and I need to through the details in connect/protocol as that seems
> to be a lot of new code, but hides the complexity very well.
> 
> > +       register_allowed_protocol_version(protocol_v2);
> > +       register_allowed_protocol_version(protocol_v1);
> > +       register_allowed_protocol_version(protocol_v0);
> 
> Would it make sense to have a
> 
>     register_allowed_protocol_versions(protocol_v2, protocol_v1,
> protocol_v0, NULL);
> 
> similar to argv_array_pushl  or would that be overengineered?

Hmm, well it wouldn't be quite as clean as the argv_* versions, since we
can't take the pointer of an enum value, so we don't have a good
stop-value for the va_list. I suppose we could use
protocol_unknown_version, but that seems unintuitive to me. We could
also pass in an explicit argument count, but... ugh.

Am I missing some other way to do this cleanly? I'll admit I'm not very
familiar with va_lists.

> > @@ -1085,10 +1085,9 @@ static struct child_process *git_connect_git(int fd[2], char *hostandport,
> >                     target_host, 0);
> >
> >         /* If using a new version put that stuff here after a second null byte */
> > -       if (version > 0) {
> > +       if (strcmp(version_advert->buf, "version=0")) {
> >                 strbuf_addch(&request, '\0');
> > -               strbuf_addf(&request, "version=%d%c",
> > -                           version, '\0');
> > +               strbuf_addf(&request, "%s%c", version_advert->buf, '\0');
> 
> It's a bit unfortunate to pass around the string, but reading through
> the following
> lines seems like it is easiest.
> 
> 
> > @@ -1226,16 +1226,10 @@ struct child_process *git_connect(int fd[2], const char *url,
> >  {
> >         char *hostandport, *path;
> >         struct child_process *conn;
> > +       struct strbuf version_advert = STRBUF_INIT;
> >         enum protocol protocol;
> > -       enum protocol_version version = get_protocol_version_config();
> >
> > -       /*
> > -        * NEEDSWORK: If we are trying to use protocol v2 and we are planning
> > -        * to perform a push, then fallback to v0 since the client doesn't know
> > -        * how to push yet using v2.
> > -        */
> > -       if (version == protocol_v2 && !strcmp("git-receive-pack", prog))
> > -               version = protocol_v0;
> > +       get_client_protocol_version_advertisement(&version_advert);
> 
> Nice to have this special casing gone!
> 
> > diff --git a/protocol.c b/protocol.c
> > index 5e636785d1..f788269c47 100644
> > +
> > +static const char protocol_v0_string[] = "0";
> > +static const char protocol_v1_string[] = "1";
> > +static const char protocol_v2_string[] = "2";
> > +
> ...
> > +/* Return the text representation of a wire protocol version. */
> > +static const char *format_protocol_version(enum protocol_version version)
> > +{
> > +       switch (version) {
> > +       case protocol_v0:
> > +               return protocol_v0_string;
> > +       case protocol_v1:
> > +               return protocol_v1_string;
> > +       case protocol_v2:
> > +               return protocol_v2_string;
> > +       case protocol_unknown_version:
> > +               die(_("Unrecognized protocol version"));
> > +       }
> > +       die(_("Unrecognized protocol_version"));
> > +}
> 
> Up to now we have the textual representation that can easily
> be constructed from protocol_version by using e.g.
>     sprintf(buf, "%d", version);
> which is why I initially thought this could be worded way
> shorter, but I guess this is more future proof?

Yeah, my understanding is that we don't want to assume that version
identifiers will always be simple integers. We could get away with
sprintf() for now, but I figured I didn't want to cause future pain
if/when the version identifiers become complex.

> > +
> >  enum protocol_version get_protocol_version_config(void)
> >  {
> >         const char *value;
> > @@ -30,6 +55,79 @@ enum protocol_version get_protocol_version_config(void)
> >         return protocol_v0;
> >  }
> >
> > +void register_allowed_protocol_version(enum protocol_version version)
> > +{
> > +       if (have_advertised_versions_already)
> > +               error(_("attempting to register an allowed protocol version after advertisement"));
> 
> This would be a
>     BUG(...)
> instead?

Ack, will fix in the next version.

Re: [PATCH v2 1/1] protocol: advertise multiple supported versions

[Stefan Beller]

> > similar to argv_array_pushl  or would that be overengineered?

> Am I missing some other way to do this cleanly? I'll admit I'm not very
> familiar with va_lists.

Ah, you're right. By not passing pointers (and I am unfamiliar
with va_lists, too), this was a moot suggestion.
>
> Yeah, my understanding is that we don't want to assume that version
> identifiers will always be simple integers. We could get away with
> sprintf() for now, but I figured I didn't want to cause future pain
> if/when the version identifiers become complex.

Makes sense.

Stefan

[PATCH v3 0/1] Advertise multiple supported proto versions

[steadmon]

This is a minor iteration on v2, to change an error message to a BUG.

Josh Steadmon (1):
  protocol: advertise multiple supported versions

 builtin/archive.c      |   3 ++
 builtin/clone.c        |   4 ++
 builtin/fetch-pack.c   |   4 ++
 builtin/fetch.c        |   5 ++
 builtin/ls-remote.c    |   5 ++
 builtin/pull.c         |   5 ++
 builtin/push.c         |   4 ++
 builtin/send-pack.c    |   3 ++
 connect.c              |  47 ++++++++---------
 protocol.c             | 112 ++++++++++++++++++++++++++++++++++++++---
 protocol.h             |  17 +++++++
 remote-curl.c          |  28 +++++++----
 t/t5570-git-daemon.sh  |   2 +-
 t/t5700-protocol-v1.sh |   8 +--
 t/t5702-protocol-v2.sh |  16 +++---
 transport-helper.c     |   6 +++
 16 files changed, 214 insertions(+), 55 deletions(-)

Range-diff against v2:
1:  70986cec32 ! 1:  b9968e3fb0 protocol: advertise multiple supported versions
    @@ -358,7 +358,7 @@
     +void register_allowed_protocol_version(enum protocol_version version)
     +{
     +	if (have_advertised_versions_already)
    -+		error(_("attempting to register an allowed protocol version after advertisement"));
    ++		BUG(_("attempting to register an allowed protocol version after advertisement"));
     +
     +	ALLOC_GROW(allowed_versions, nr_allowed_versions + 1,
     +		   alloc_allowed_versions);
    @@ -395,7 +395,6 @@
     +	config_version = get_protocol_version_config();
     +	if (config_version == protocol_v0) {
     +		strbuf_addstr(advert, "version=0");
    -+		trace_printf("Client advert string: %s", advert->buf);
     +		return;
     +	}
     +
    @@ -424,8 +423,6 @@
     +	for (int i = 1; i < tmp_nr; i++)
     +		strbuf_addf(advert, ":version=%s",
     +			    format_protocol_version(tmp_allowed_versions[i]));
    -+
    -+	trace_printf("Client advert string: %s", advert->buf);
     +}
     +
      enum protocol_version determine_protocol_version_server(void)
-- 
2.19.1.930.g4563a0d9d0-goog

[PATCH v3 1/1] protocol: advertise multiple supported versions

[steadmon]

Currently the client advertises that it supports the wire protocol
version set in the protocol.version config. However, not all services
support the same set of protocol versions. When connecting to
git-receive-pack, the client automatically downgrades to v0 if
config.protocol is set to v2, but this check is not performed for other
services.

This patch creates a protocol version registry. Individual operations
register all the protocol versions they support prior to communicating
with a server. Versions should be listed in preference order; the
version specified in protocol.version will automatically be moved to the
front of the registry.

The protocol version registry is passed to remote helpers via the
GIT_PROTOCOL environment variable.

Clients now advertise the full list of registered versions. Servers
select the first recognized version from this advertisement.

Signed-off-by: Josh Steadmon <steadmon@google.com>
---
 builtin/archive.c      |   3 ++
 builtin/clone.c        |   4 ++
 builtin/fetch-pack.c   |   4 ++
 builtin/fetch.c        |   5 ++
 builtin/ls-remote.c    |   5 ++
 builtin/pull.c         |   5 ++
 builtin/push.c         |   4 ++
 builtin/send-pack.c    |   3 ++
 connect.c              |  47 ++++++++---------
 protocol.c             | 112 ++++++++++++++++++++++++++++++++++++++---
 protocol.h             |  17 +++++++
 remote-curl.c          |  28 +++++++----
 t/t5570-git-daemon.sh  |   2 +-
 t/t5700-protocol-v1.sh |   8 +--
 t/t5702-protocol-v2.sh |  16 +++---
 transport-helper.c     |   6 +++
 16 files changed, 214 insertions(+), 55 deletions(-)

diff --git a/builtin/archive.c b/builtin/archive.c
index e74f675390..8adb9f381b 100644
--- a/builtin/archive.c
+++ b/builtin/archive.c
@@ -8,6 +8,7 @@
 #include "transport.h"
 #include "parse-options.h"
 #include "pkt-line.h"
+#include "protocol.h"
 #include "sideband.h"
 
 static void create_output_file(const char *output_file)
@@ -94,6 +95,8 @@ int cmd_archive(int argc, const char **argv, const char *prefix)
 		OPT_END()
 	};
 
+	register_allowed_protocol_version(protocol_v0);
+
 	argc = parse_options(argc, argv, prefix, local_opts, NULL,
 			     PARSE_OPT_KEEP_ALL);
 
diff --git a/builtin/clone.c b/builtin/clone.c
index fd2c3ef090..1651a950b6 100644
--- a/builtin/clone.c
+++ b/builtin/clone.c
@@ -900,6 +900,10 @@ int cmd_clone(int argc, const char **argv, const char *prefix)
 	struct refspec rs = REFSPEC_INIT_FETCH;
 	struct argv_array ref_prefixes = ARGV_ARRAY_INIT;
 
+	register_allowed_protocol_version(protocol_v2);
+	register_allowed_protocol_version(protocol_v1);
+	register_allowed_protocol_version(protocol_v0);
+
 	fetch_if_missing = 0;
 
 	packet_trace_identity("clone");
diff --git a/builtin/fetch-pack.c b/builtin/fetch-pack.c
index 1a1bc63566..cba935e4d3 100644
--- a/builtin/fetch-pack.c
+++ b/builtin/fetch-pack.c
@@ -57,6 +57,10 @@ int cmd_fetch_pack(int argc, const char **argv, const char *prefix)
 
 	fetch_if_missing = 0;
 
+	register_allowed_protocol_version(protocol_v2);
+	register_allowed_protocol_version(protocol_v1);
+	register_allowed_protocol_version(protocol_v0);
+
 	packet_trace_identity("fetch-pack");
 
 	memset(&args, 0, sizeof(args));
diff --git a/builtin/fetch.c b/builtin/fetch.c
index 61bec5d213..2a20cf8bfd 100644
--- a/builtin/fetch.c
+++ b/builtin/fetch.c
@@ -21,6 +21,7 @@
 #include "argv-array.h"
 #include "utf8.h"
 #include "packfile.h"
+#include "protocol.h"
 #include "list-objects-filter-options.h"
 
 static const char * const builtin_fetch_usage[] = {
@@ -1476,6 +1477,10 @@ int cmd_fetch(int argc, const char **argv, const char *prefix)
 	int prune_tags_ok = 1;
 	struct argv_array argv_gc_auto = ARGV_ARRAY_INIT;
 
+	register_allowed_protocol_version(protocol_v2);
+	register_allowed_protocol_version(protocol_v1);
+	register_allowed_protocol_version(protocol_v0);
+
 	packet_trace_identity("fetch");
 
 	fetch_if_missing = 0;
diff --git a/builtin/ls-remote.c b/builtin/ls-remote.c
index 1a25df7ee1..ea685e8bb9 100644
--- a/builtin/ls-remote.c
+++ b/builtin/ls-remote.c
@@ -1,5 +1,6 @@
 #include "builtin.h"
 #include "cache.h"
+#include "protocol.h"
 #include "transport.h"
 #include "ref-filter.h"
 #include "remote.h"
@@ -80,6 +81,10 @@ int cmd_ls_remote(int argc, const char **argv, const char *prefix)
 
 	memset(&ref_array, 0, sizeof(ref_array));
 
+	register_allowed_protocol_version(protocol_v2);
+	register_allowed_protocol_version(protocol_v1);
+	register_allowed_protocol_version(protocol_v0);
+
 	argc = parse_options(argc, argv, prefix, options, ls_remote_usage,
 			     PARSE_OPT_STOP_AT_NON_OPTION);
 	dest = argv[0];
diff --git a/builtin/pull.c b/builtin/pull.c
index 681c127a07..cb64146834 100644
--- a/builtin/pull.c
+++ b/builtin/pull.c
@@ -9,6 +9,7 @@
 #include "config.h"
 #include "builtin.h"
 #include "parse-options.h"
+#include "protocol.h"
 #include "exec-cmd.h"
 #include "run-command.h"
 #include "sha1-array.h"
@@ -849,6 +850,10 @@ int cmd_pull(int argc, const char **argv, const char *prefix)
 	struct object_id rebase_fork_point;
 	int autostash;
 
+	register_allowed_protocol_version(protocol_v2);
+	register_allowed_protocol_version(protocol_v1);
+	register_allowed_protocol_version(protocol_v0);
+
 	if (!getenv("GIT_REFLOG_ACTION"))
 		set_reflog_message(argc, argv);
 
diff --git a/builtin/push.c b/builtin/push.c
index d09a42062c..10d8abe829 100644
--- a/builtin/push.c
+++ b/builtin/push.c
@@ -10,6 +10,7 @@
 #include "remote.h"
 #include "transport.h"
 #include "parse-options.h"
+#include "protocol.h"
 #include "submodule.h"
 #include "submodule-config.h"
 #include "send-pack.h"
@@ -587,6 +588,9 @@ int cmd_push(int argc, const char **argv, const char *prefix)
 		OPT_END()
 	};
 
+	register_allowed_protocol_version(protocol_v1);
+	register_allowed_protocol_version(protocol_v0);
+
 	packet_trace_identity("push");
 	git_config(git_push_config, &flags);
 	argc = parse_options(argc, argv, prefix, options, push_usage, 0);
diff --git a/builtin/send-pack.c b/builtin/send-pack.c
index 8e3c7490f7..f48bd1306b 100644
--- a/builtin/send-pack.c
+++ b/builtin/send-pack.c
@@ -184,6 +184,9 @@ int cmd_send_pack(int argc, const char **argv, const char *prefix)
 		OPT_END()
 	};
 
+	register_allowed_protocol_version(protocol_v1);
+	register_allowed_protocol_version(protocol_v0);
+
 	git_config(send_pack_config, NULL);
 	argc = parse_options(argc, argv, prefix, options, send_pack_usage, 0);
 	if (argc > 0) {
diff --git a/connect.c b/connect.c
index 94547e5056..a170ba1e90 100644
--- a/connect.c
+++ b/connect.c
@@ -1046,7 +1046,7 @@ static enum ssh_variant determine_ssh_variant(const char *ssh_command,
  */
 static struct child_process *git_connect_git(int fd[2], char *hostandport,
 					     const char *path, const char *prog,
-					     enum protocol_version version,
+					     const struct strbuf *version_advert,
 					     int flags)
 {
 	struct child_process *conn;
@@ -1085,10 +1085,9 @@ static struct child_process *git_connect_git(int fd[2], char *hostandport,
 		    target_host, 0);
 
 	/* If using a new version put that stuff here after a second null byte */
-	if (version > 0) {
+	if (strcmp(version_advert->buf, "version=0")) {
 		strbuf_addch(&request, '\0');
-		strbuf_addf(&request, "version=%d%c",
-			    version, '\0');
+		strbuf_addf(&request, "%s%c", version_advert->buf, '\0');
 	}
 
 	packet_write(fd[1], request.buf, request.len);
@@ -1104,14 +1103,14 @@ static struct child_process *git_connect_git(int fd[2], char *hostandport,
  */
 static void push_ssh_options(struct argv_array *args, struct argv_array *env,
 			     enum ssh_variant variant, const char *port,
-			     enum protocol_version version, int flags)
+			     const struct strbuf *version_advert, int flags)
 {
 	if (variant == VARIANT_SSH &&
-	    version > 0) {
+	    strcmp(version_advert->buf, "version=0")) {
 		argv_array_push(args, "-o");
 		argv_array_push(args, "SendEnv=" GIT_PROTOCOL_ENVIRONMENT);
-		argv_array_pushf(env, GIT_PROTOCOL_ENVIRONMENT "=version=%d",
-				 version);
+		argv_array_pushf(env, GIT_PROTOCOL_ENVIRONMENT "=%s",
+				 version_advert->buf);
 	}
 
 	if (flags & CONNECT_IPV4) {
@@ -1164,7 +1163,7 @@ static void push_ssh_options(struct argv_array *args, struct argv_array *env,
 
 /* Prepare a child_process for use by Git's SSH-tunneled transport. */
 static void fill_ssh_args(struct child_process *conn, const char *ssh_host,
-			  const char *port, enum protocol_version version,
+			  const char *port, const struct strbuf *version_advert,
 			  int flags)
 {
 	const char *ssh;
@@ -1198,15 +1197,16 @@ static void fill_ssh_args(struct child_process *conn, const char *ssh_host,
 
 		argv_array_push(&detect.args, ssh);
 		argv_array_push(&detect.args, "-G");
-		push_ssh_options(&detect.args, &detect.env_array,
-				 VARIANT_SSH, port, version, flags);
+		push_ssh_options(&detect.args, &detect.env_array, VARIANT_SSH,
+				 port, version_advert, flags);
 		argv_array_push(&detect.args, ssh_host);
 
 		variant = run_command(&detect) ? VARIANT_SIMPLE : VARIANT_SSH;
 	}
 
 	argv_array_push(&conn->args, ssh);
-	push_ssh_options(&conn->args, &conn->env_array, variant, port, version, flags);
+	push_ssh_options(&conn->args, &conn->env_array, variant, port,
+			 version_advert, flags);
 	argv_array_push(&conn->args, ssh_host);
 }
 
@@ -1226,16 +1226,10 @@ struct child_process *git_connect(int fd[2], const char *url,
 {
 	char *hostandport, *path;
 	struct child_process *conn;
+	struct strbuf version_advert = STRBUF_INIT;
 	enum protocol protocol;
-	enum protocol_version version = get_protocol_version_config();
 
-	/*
-	 * NEEDSWORK: If we are trying to use protocol v2 and we are planning
-	 * to perform a push, then fallback to v0 since the client doesn't know
-	 * how to push yet using v2.
-	 */
-	if (version == protocol_v2 && !strcmp("git-receive-pack", prog))
-		version = protocol_v0;
+	get_client_protocol_version_advertisement(&version_advert);
 
 	/* Without this we cannot rely on waitpid() to tell
 	 * what happened to our children.
@@ -1250,7 +1244,8 @@ struct child_process *git_connect(int fd[2], const char *url,
 		printf("Diag: path=%s\n", path ? path : "NULL");
 		conn = NULL;
 	} else if (protocol == PROTO_GIT) {
-		conn = git_connect_git(fd, hostandport, path, prog, version, flags);
+		conn = git_connect_git(fd, hostandport, path, prog,
+				       &version_advert, flags);
 	} else {
 		struct strbuf cmd = STRBUF_INIT;
 		const char *const *var;
@@ -1293,12 +1288,14 @@ struct child_process *git_connect(int fd[2], const char *url,
 				strbuf_release(&cmd);
 				return NULL;
 			}
-			fill_ssh_args(conn, ssh_host, port, version, flags);
+			fill_ssh_args(conn, ssh_host, port, &version_advert,
+				      flags);
 		} else {
 			transport_check_allowed("file");
-			if (version > 0) {
-				argv_array_pushf(&conn->env_array, GIT_PROTOCOL_ENVIRONMENT "=version=%d",
-						 version);
+			if (strcmp(version_advert.buf, "version=0")) {
+				argv_array_pushf(&conn->env_array,
+						 GIT_PROTOCOL_ENVIRONMENT "=%s",
+						 version_advert.buf);
 			}
 		}
 		argv_array_push(&conn->args, cmd.buf);
diff --git a/protocol.c b/protocol.c
index 5e636785d1..54d2ab991b 100644
--- a/protocol.c
+++ b/protocol.c
@@ -2,18 +2,43 @@
 #include "config.h"
 #include "protocol.h"
 
+static enum protocol_version *allowed_versions;
+static int nr_allowed_versions;
+static int alloc_allowed_versions;
+static int have_advertised_versions_already = 0;
+
+static const char protocol_v0_string[] = "0";
+static const char protocol_v1_string[] = "1";
+static const char protocol_v2_string[] = "2";
+
 static enum protocol_version parse_protocol_version(const char *value)
 {
-	if (!strcmp(value, "0"))
+	if (!strcmp(value, protocol_v0_string))
 		return protocol_v0;
-	else if (!strcmp(value, "1"))
+	else if (!strcmp(value, protocol_v1_string))
 		return protocol_v1;
-	else if (!strcmp(value, "2"))
+	else if (!strcmp(value, protocol_v2_string))
 		return protocol_v2;
 	else
 		return protocol_unknown_version;
 }
 
+/* Return the text representation of a wire protocol version. */
+static const char *format_protocol_version(enum protocol_version version)
+{
+	switch (version) {
+	case protocol_v0:
+		return protocol_v0_string;
+	case protocol_v1:
+		return protocol_v1_string;
+	case protocol_v2:
+		return protocol_v2_string;
+	case protocol_unknown_version:
+		die(_("Unrecognized protocol version"));
+	}
+	die(_("Unrecognized protocol_version"));
+}
+
 enum protocol_version get_protocol_version_config(void)
 {
 	const char *value;
@@ -30,6 +55,76 @@ enum protocol_version get_protocol_version_config(void)
 	return protocol_v0;
 }
 
+void register_allowed_protocol_version(enum protocol_version version)
+{
+	if (have_advertised_versions_already)
+		BUG(_("attempting to register an allowed protocol version after advertisement"));
+
+	ALLOC_GROW(allowed_versions, nr_allowed_versions + 1,
+		   alloc_allowed_versions);
+	allowed_versions[nr_allowed_versions++] = version;
+}
+
+void register_allowed_protocol_versions_from_env(void)
+{
+	const char *git_protocol = getenv(GIT_PROTOCOL_ENVIRONMENT);
+	const char *version_str;
+	struct string_list version_list = STRING_LIST_INIT_DUP;
+	struct string_list_item *version;
+
+	if (!git_protocol)
+		return;
+
+	string_list_split(&version_list, git_protocol, ':', -1);
+	for_each_string_list_item(version, &version_list) {
+		if (skip_prefix(version->string, "version=", &version_str))
+			register_allowed_protocol_version(
+				parse_protocol_version(version_str));
+	}
+	string_list_clear(&version_list, 0);
+}
+
+void get_client_protocol_version_advertisement(struct strbuf *advert)
+{
+	int tmp_nr = nr_allowed_versions;
+	enum protocol_version *tmp_allowed_versions, config_version;
+	strbuf_reset(advert);
+
+	have_advertised_versions_already = 1;
+
+	config_version = get_protocol_version_config();
+	if (config_version == protocol_v0) {
+		strbuf_addstr(advert, "version=0");
+		return;
+	}
+
+	if (tmp_nr > 0) {
+		ALLOC_ARRAY(tmp_allowed_versions, tmp_nr);
+		copy_array(tmp_allowed_versions, allowed_versions, tmp_nr,
+			   sizeof(enum protocol_version));
+	} else {
+		ALLOC_ARRAY(tmp_allowed_versions, 1);
+		tmp_nr = 1;
+		tmp_allowed_versions[0] = config_version;
+	}
+
+	if (tmp_allowed_versions[0] != config_version)
+		for (int i = 1; i < nr_allowed_versions; i++)
+			if (tmp_allowed_versions[i] == config_version) {
+				enum protocol_version swap =
+					tmp_allowed_versions[0];
+				tmp_allowed_versions[0] =
+					tmp_allowed_versions[i];
+				tmp_allowed_versions[i] = swap;
+			}
+
+	strbuf_addf(advert, "version=%s",
+		    format_protocol_version(tmp_allowed_versions[0]));
+	for (int i = 1; i < tmp_nr; i++)
+		strbuf_addf(advert, ":version=%s",
+			    format_protocol_version(tmp_allowed_versions[i]));
+}
+
 enum protocol_version determine_protocol_version_server(void)
 {
 	const char *git_protocol = getenv(GIT_PROTOCOL_ENVIRONMENT);
@@ -38,9 +133,10 @@ enum protocol_version determine_protocol_version_server(void)
 	/*
 	 * Determine which protocol version the client has requested.  Since
 	 * multiple 'version' keys can be sent by the client, indicating that
-	 * the client is okay to speak any of them, select the greatest version
-	 * that the client has requested.  This is due to the assumption that
-	 * the most recent protocol version will be the most state-of-the-art.
+	 * the client is okay to speak any of them, select the first
+	 * recognizable version that the client has requested.  This is due to
+	 * the assumption that the protocol versions will be listed in
+	 * preference order.
 	 */
 	if (git_protocol) {
 		struct string_list list = STRING_LIST_INIT_DUP;
@@ -53,8 +149,10 @@ enum protocol_version determine_protocol_version_server(void)
 
 			if (skip_prefix(item->string, "version=", &value)) {
 				v = parse_protocol_version(value);
-				if (v > version)
+				if (v != protocol_unknown_version) {
 					version = v;
+					break;
+				}
 			}
 		}
 
diff --git a/protocol.h b/protocol.h
index 2ad35e433c..b67b2259de 100644
--- a/protocol.h
+++ b/protocol.h
@@ -16,6 +16,23 @@ enum protocol_version {
  */
 extern enum protocol_version get_protocol_version_config(void);
 
+/*
+ * Register an allowable protocol version for a given operation. Registration
+ * must occur before attempting to advertise a version to a server process.
+ */
+extern void register_allowed_protocol_version(enum protocol_version version);
+
+/*
+ * Register allowable protocol versions from the GIT_PROTOCOL environment var.
+ */
+extern void register_allowed_protocol_versions_from_env(void);
+
+/*
+ * Fill a strbuf with a version advertisement string suitable for use in the
+ * GIT_PROTOCOL environment variable or similar version negotiation field.
+ */
+extern void get_client_protocol_version_advertisement(struct strbuf *advert);
+
 /*
  * Used by a server to determine which protocol version should be used based on
  * a client's request, communicated via the 'GIT_PROTOCOL' environment variable
diff --git a/remote-curl.c b/remote-curl.c
index fb28309e85..537f8c7aac 100644
--- a/remote-curl.c
+++ b/remote-curl.c
@@ -330,6 +330,20 @@ static int get_protocol_http_header(enum protocol_version version,
 	return 0;
 }
 
+static int get_client_protocol_http_header(const struct strbuf *version_advert,
+					   struct strbuf *header)
+{
+	if (version_advert->len > 0 &&
+	    strcmp(version_advert->buf, "version=0")) {
+		strbuf_addf(header, GIT_PROTOCOL_HEADER ": %s",
+			    version_advert->buf);
+
+		return 1;
+	}
+
+	return 0;
+}
+
 static struct discovery *discover_refs(const char *service, int for_push)
 {
 	struct strbuf exp = STRBUF_INIT;
@@ -339,11 +353,11 @@ static struct discovery *discover_refs(const char *service, int for_push)
 	struct strbuf refs_url = STRBUF_INIT;
 	struct strbuf effective_url = STRBUF_INIT;
 	struct strbuf protocol_header = STRBUF_INIT;
+	struct strbuf version_advert = STRBUF_INIT;
 	struct string_list extra_headers = STRING_LIST_INIT_DUP;
 	struct discovery *last = last_discovery;
 	int http_ret, maybe_smart = 0;
 	struct http_get_options http_options;
-	enum protocol_version version = get_protocol_version_config();
 
 	if (last && !strcmp(service, last->service))
 		return last;
@@ -360,16 +374,10 @@ static struct discovery *discover_refs(const char *service, int for_push)
 		strbuf_addf(&refs_url, "service=%s", service);
 	}
 
-	/*
-	 * NEEDSWORK: If we are trying to use protocol v2 and we are planning
-	 * to perform a push, then fallback to v0 since the client doesn't know
-	 * how to push yet using v2.
-	 */
-	if (version == protocol_v2 && !strcmp("git-receive-pack", service))
-		version = protocol_v0;
+	get_client_protocol_version_advertisement(&version_advert);
 
 	/* Add the extra Git-Protocol header */
-	if (get_protocol_http_header(version, &protocol_header))
+	if (get_client_protocol_http_header(&version_advert, &protocol_header))
 		string_list_append(&extra_headers, protocol_header.buf);
 
 	memset(&http_options, 0, sizeof(http_options));
@@ -1327,6 +1335,8 @@ int cmd_main(int argc, const char **argv)
 	struct strbuf buf = STRBUF_INIT;
 	int nongit;
 
+	register_allowed_protocol_versions_from_env();
+
 	setup_git_directory_gently(&nongit);
 	if (argc < 2) {
 		error("remote-curl: usage: git remote-curl <remote> [<url>]");
diff --git a/t/t5570-git-daemon.sh b/t/t5570-git-daemon.sh
index 7466aad111..d528e91630 100755
--- a/t/t5570-git-daemon.sh
+++ b/t/t5570-git-daemon.sh
@@ -186,7 +186,7 @@ test_expect_success 'hostname cannot break out of directory' '
 test_expect_success 'daemon log records all attributes' '
 	cat >expect <<-\EOF &&
 	Extended attribute "host": localhost
-	Extended attribute "protocol": version=1
+	Extended attribute "protocol": version=1:version=2:version=0
 	EOF
 	>daemon.log &&
 	GIT_OVERRIDE_VIRTUAL_HOST=localhost \
diff --git a/t/t5700-protocol-v1.sh b/t/t5700-protocol-v1.sh
index ba86a44eb1..2e56c79233 100755
--- a/t/t5700-protocol-v1.sh
+++ b/t/t5700-protocol-v1.sh
@@ -26,7 +26,7 @@ test_expect_success 'clone with git:// using protocol v1' '
 	test_cmp expect actual &&
 
 	# Client requested to use protocol v1
-	grep "clone> .*\\\0\\\0version=1\\\0$" log &&
+	grep "clone> .*\\\0\\\0version=1.*\\\0$" log &&
 	# Server responded using protocol v1
 	grep "clone< version 1" log
 '
@@ -42,7 +42,7 @@ test_expect_success 'fetch with git:// using protocol v1' '
 	test_cmp expect actual &&
 
 	# Client requested to use protocol v1
-	grep "fetch> .*\\\0\\\0version=1\\\0$" log &&
+	grep "fetch> .*\\\0\\\0version=1.*\\\0$" log &&
 	# Server responded using protocol v1
 	grep "fetch< version 1" log
 '
@@ -56,7 +56,7 @@ test_expect_success 'pull with git:// using protocol v1' '
 	test_cmp expect actual &&
 
 	# Client requested to use protocol v1
-	grep "fetch> .*\\\0\\\0version=1\\\0$" log &&
+	grep "fetch> .*\\\0\\\0version=1.*\\\0$" log &&
 	# Server responded using protocol v1
 	grep "fetch< version 1" log
 '
@@ -74,7 +74,7 @@ test_expect_success 'push with git:// using protocol v1' '
 	test_cmp expect actual &&
 
 	# Client requested to use protocol v1
-	grep "push> .*\\\0\\\0version=1\\\0$" log &&
+	grep "push> .*\\\0\\\0version=1.*\\\0$" log &&
 	# Server responded using protocol v1
 	grep "push< version 1" log
 '
diff --git a/t/t5702-protocol-v2.sh b/t/t5702-protocol-v2.sh
index 3beeed4546..78c17c25e4 100755
--- a/t/t5702-protocol-v2.sh
+++ b/t/t5702-protocol-v2.sh
@@ -24,7 +24,7 @@ test_expect_success 'list refs with git:// using protocol v2' '
 		ls-remote --symref "$GIT_DAEMON_URL/parent" >actual &&
 
 	# Client requested to use protocol v2
-	grep "git> .*\\\0\\\0version=2\\\0$" log &&
+	grep "git> .*\\\0\\\0version=2.*\\\0$" log &&
 	# Server responded using protocol v2
 	grep "git< version 2" log &&
 
@@ -56,7 +56,7 @@ test_expect_success 'clone with git:// using protocol v2' '
 	test_cmp expect actual &&
 
 	# Client requested to use protocol v2
-	grep "clone> .*\\\0\\\0version=2\\\0$" log &&
+	grep "clone> .*\\\0\\\0version=2.*\\\0$" log &&
 	# Server responded using protocol v2
 	grep "clone< version 2" log
 '
@@ -74,7 +74,7 @@ test_expect_success 'fetch with git:// using protocol v2' '
 	test_cmp expect actual &&
 
 	# Client requested to use protocol v2
-	grep "fetch> .*\\\0\\\0version=2\\\0$" log &&
+	grep "fetch> .*\\\0\\\0version=2.*\\\0$" log &&
 	# Server responded using protocol v2
 	grep "fetch< version 2" log
 '
@@ -90,7 +90,7 @@ test_expect_success 'pull with git:// using protocol v2' '
 	test_cmp expect actual &&
 
 	# Client requested to use protocol v2
-	grep "fetch> .*\\\0\\\0version=2\\\0$" log &&
+	grep "fetch> .*\\\0\\\0version=2.*\\\0$" log &&
 	# Server responded using protocol v2
 	grep "fetch< version 2" log
 '
@@ -476,7 +476,7 @@ test_expect_success 'push with http:// and a config of v2 does not request v2' '
 	test_when_finished "rm -f log" &&
 	# Till v2 for push is designed, make sure that if a client has
 	# protocol.version configured to use v2, that the client instead falls
-	# back and uses v0.
+	# back to previous versions.
 
 	test_commit -C http_child three &&
 
@@ -489,10 +489,8 @@ test_expect_success 'push with http:// and a config of v2 does not request v2' '
 	git -C "$HTTPD_DOCUMENT_ROOT_PATH/http_parent" log -1 --format=%s client_branch >expect &&
 	test_cmp expect actual &&
 
-	# Client didnt request to use protocol v2
-	! grep "Git-Protocol: version=2" log &&
-	# Server didnt respond using protocol v2
-	! grep "git< version 2" log
+	# Server responded with version 1
+	grep "git< version 1" log
 '
 
 
diff --git a/transport-helper.c b/transport-helper.c
index 143ca008c8..ac1937f1e1 100644
--- a/transport-helper.c
+++ b/transport-helper.c
@@ -105,6 +105,7 @@ static struct child_process *get_helper(struct transport *transport)
 {
 	struct helper_data *data = transport->data;
 	struct strbuf buf = STRBUF_INIT;
+	struct strbuf version_advert = STRBUF_INIT;
 	struct child_process *helper;
 	int duped;
 	int code;
@@ -127,6 +128,11 @@ static struct child_process *get_helper(struct transport *transport)
 		argv_array_pushf(&helper->env_array, "%s=%s",
 				 GIT_DIR_ENVIRONMENT, get_git_dir());
 
+	get_client_protocol_version_advertisement(&version_advert);
+	if (version_advert.len > 0)
+		argv_array_pushf(&helper->env_array, "%s=%s",
+				 GIT_PROTOCOL_ENVIRONMENT, version_advert.buf);
+
 	code = start_command(helper);
 	if (code < 0 && errno == ENOENT)
 		die(_("unable to find remote helper for '%s'"), data->name);
-- 
2.19.1.930.g4563a0d9d0-goog

Re: [PATCH v3 1/1] protocol: advertise multiple supported versions

[Stefan Beller]

On Mon, Nov 12, 2018 at 1:49 PM <steadmon@google.com> wrote:
>
> Currently the client advertises that it supports the wire protocol
> version set in the protocol.version config. However, not all services
> support the same set of protocol versions. When connecting to
> git-receive-pack, the client automatically downgrades to v0 if
> config.protocol is set to v2, but this check is not performed for other
> services.
>
> This patch creates a protocol version registry. Individual operations
> register all the protocol versions they support prior to communicating
> with a server. Versions should be listed in preference order; the
> version specified in protocol.version will automatically be moved to the
> front of the registry.
>
> The protocol version registry is passed to remote helpers via the
> GIT_PROTOCOL environment variable.
>
> Clients now advertise the full list of registered versions. Servers
> select the first recognized version from this advertisement.
>
> Signed-off-by: Josh Steadmon <steadmon@google.com>

Thanks for resending this patch!

+cc Jonathan Tan, who works a lot on the protocol side of things.

> +void register_allowed_protocol_version(enum protocol_version version)
> +{
> +       if (have_advertised_versions_already)
> +               BUG(_("attempting to register an allowed protocol version after advertisement"));

If this is a real BUG (due to wrong program flow) instead of bad user input,
we would not want to burden the translators with this message.

If it is a message that the user is intended to see upon bad input
we'd rather go with die(_("translatable text here"));


> diff --git a/protocol.h b/protocol.h
> index 2ad35e433c..b67b2259de 100644
> --- a/protocol.h
> +++ b/protocol.h
> @@ -16,6 +16,23 @@ enum protocol_version {
>   */
>  extern enum protocol_version get_protocol_version_config(void);
>
> +/*
> + * Register an allowable protocol version for a given operation. Registration
> + * must occur before attempting to advertise a version to a server process.
> + */
> +extern void register_allowed_protocol_version(enum protocol_version version);

We keep extern here as to imitate the file-local style, although
Documentation/CodingGuidelines would prefer to not have the extern keywords.
Okay.

Bonus points for converting the file to omit all extern keywords in a
resend. :-)
(I think there is no other series currently in flight touching this header file,
so it is safe to convert it "while at it", `git log --grep "while at
it"` is shorter
than expected.)

All that said, it's only nits that I contributed to this code review;
the code/design
looks good to me, and if I were maintainer I'd include it as-is, as it
fixes a long(ish)
standing protocol error.

Thanks,
Stefan

Re* [PATCH v3 1/1] protocol: advertise multiple supported versions

[Junio C Hamano]

Stefan Beller <sbeller@google.com> writes:

>> +       if (have_advertised_versions_already)
>> +               BUG(_("attempting to register an allowed protocol version after advertisement"));
>
> If this is a real BUG (due to wrong program flow) instead of bad user input,
> we would not want to burden the translators with this message.
>
> If it is a message that the user is intended to see upon bad input
> we'd rather go with die(_("translatable text here"));

Yeah, good suggestion.  

Perhaps we should spell it out in docstrings for BUG/die with the
above rationale.  A weatherbaloon patch follows.

 git-compat-util.h | 22 +++++++++++++++++++++-
 1 file changed, 21 insertions(+), 1 deletion(-)

diff --git a/git-compat-util.h b/git-compat-util.h
index 96a3f86d8e..a1cf68cbbc 100644
--- a/git-compat-util.h
+++ b/git-compat-util.h
@@ -420,7 +420,16 @@ static inline char *git_find_last_dir_sep(const char *path)
 
 struct strbuf;
 
-/* General helper functions */
+/*
+ * General helper functions
+ *
+ * Note that these are to produce end-user facing messages, and most
+ * of them should be marked for translation (the exception is
+ * messages generated to be sent over the wire---as we currently do not
+ * have a mechanism to notice and honor locale settings used on the
+ * other end, leave them untranslated.  We should *not* send messages
+ * that are localized for our end).
+ */
 extern void vreportf(const char *prefix, const char *err, va_list params);
 extern NORETURN void usage(const char *err);
 extern NORETURN void usagef(const char *err, ...) __attribute__((format (printf, 1, 2)));
@@ -1142,6 +1151,17 @@ static inline int regexec_buf(const regex_t *preg, const char *buf, size_t size,
 /* usage.c: only to be used for testing BUG() implementation (see test-tool) */
 extern int BUG_exit_code;
 
+/*
+ * BUG(fmt, ...) is to be used when the problem of reaching that point
+ * in code can only be caused by a program error.  To abort with a
+ * message to report impossible-to-continue condition due to external
+ * factors like end-user input, environment settings, data it was fed
+ * over the wire, use die(_(fmt),...).
+ * 
+ * Note that the message from BUG() should not be marked for
+ * translation while the message from die() is in general end-user
+ * facing and should be marked for translation.
+ */
 #ifdef HAVE_VARIADIC_MACROS
 __attribute__((format (printf, 3, 4))) NORETURN
 void BUG_fl(const char *file, int line, const char *fmt, ...);

Re: Re* [PATCH v3 1/1] protocol: advertise multiple supported versions

[Stefan Beller]

On Mon, Nov 12, 2018 at 7:24 PM Junio C Hamano <gitster@pobox.com> wrote:
>
> Stefan Beller <sbeller@google.com> writes:
>
> >> +       if (have_advertised_versions_already)
> >> +               BUG(_("attempting to register an allowed protocol version after advertisement"));
> >
> > If this is a real BUG (due to wrong program flow) instead of bad user input,
> > we would not want to burden the translators with this message.
> >
> > If it is a message that the user is intended to see upon bad input
> > we'd rather go with die(_("translatable text here"));
>
> Yeah, good suggestion.
>
> Perhaps we should spell it out in docstrings for BUG/die with the
> above rationale.  A weatherbaloon patch follows.

"Nobody reads documentation any more, we have too much of it." [1]
[1] c.f. https://quoteinvestigator.com/2014/08/29/too-crowded/

I would have hoped to have a .cocci patch in the bad style category,
i.e. disallowing the _() function inside the context of BUG().

That said, I like the patch below. Thanks for writing it.

>  git-compat-util.h | 22 +++++++++++++++++++++-
>  1 file changed, 21 insertions(+), 1 deletion(-)
>
> diff --git a/git-compat-util.h b/git-compat-util.h
> index 96a3f86d8e..a1cf68cbbc 100644
> --- a/git-compat-util.h
> +++ b/git-compat-util.h
> @@ -420,7 +420,16 @@ static inline char *git_find_last_dir_sep(const char *path)
>
>  struct strbuf;
>
> -/* General helper functions */
> +/*
> + * General helper functions
> + *
> + * Note that these are to produce end-user facing messages, and most
> + * of them should be marked for translation (the exception is
> + * messages generated to be sent over the wire---as we currently do not
> + * have a mechanism to notice and honor locale settings used on the
> + * other end, leave them untranslated.  We should *not* send messages
> + * that are localized for our end).
> + */
>  extern void vreportf(const char *prefix, const char *err, va_list params);
>  extern NORETURN void usage(const char *err);
>  extern NORETURN void usagef(const char *err, ...) __attribute__((format (printf, 1, 2)));
> @@ -1142,6 +1151,17 @@ static inline int regexec_buf(const regex_t *preg, const char *buf, size_t size,
>  /* usage.c: only to be used for testing BUG() implementation (see test-tool) */
>  extern int BUG_exit_code;
>
> +/*
> + * BUG(fmt, ...) is to be used when the problem of reaching that point
> + * in code can only be caused by a program error.  To abort with a
> + * message to report impossible-to-continue condition due to external
> + * factors like end-user input, environment settings, data it was fed
> + * over the wire, use die(_(fmt),...).
> + *
> + * Note that the message from BUG() should not be marked for
> + * translation while the message from die() is in general end-user
> + * facing and should be marked for translation.
> + */
>  #ifdef HAVE_VARIADIC_MACROS
>  __attribute__((format (printf, 3, 4))) NORETURN
>  void BUG_fl(const char *file, int line, const char *fmt, ...);

Re: Re* [PATCH v3 1/1] protocol: advertise multiple supported versions

[Junio C Hamano]

Stefan Beller <sbeller@google.com> writes:

> "Nobody reads documentation any more, we have too much of it."

Maybe, but spelling it out and writing it down, you can point at it
(or hope that other people point at it without making a bad or incorrect
rephrase).

> I would have hoped to have a .cocci patch in the bad style category,
> i.e. disallowing the _() function inside the context of BUG().
>
> That said, I like the patch below. Thanks for writing it.

That was a mere weather-baloon, and too early to thank about.
Hopefully people can pick nits and come to a final version that can
be applied.

I have a few things in it that I think may be debatable.  I am not
sure if we should talk about being careful not to send localized
strings over the wire in that comment, for example.

Re: [PATCH v3 1/1] protocol: advertise multiple supported versions

[Junio C Hamano]

steadmon@google.com writes:

> Currently the client advertises that it supports the wire protocol
> version set in the protocol.version config. However, not all services
> support the same set of protocol versions. When connecting to
> git-receive-pack, the client automatically downgrades to v0 if
> config.protocol is set to v2, but this check is not performed for other
> services.

"downgrades to v0 even if ... is set to v2" you mean?  Otherwise it
is unclear why asking for v2 leads to using v0.

> This patch creates a protocol version registry. Individual operations
> register all the protocol versions they support prior to communicating
> with a server. Versions should be listed in preference order; the
> version specified in protocol.version will automatically be moved to the
> front of the registry.
>
> The protocol version registry is passed to remote helpers via the
> GIT_PROTOCOL environment variable.
>
> Clients now advertise the full list of registered versions. Servers
> select the first recognized version from this advertisement.

Makes sense.

> +void get_client_protocol_version_advertisement(struct strbuf *advert)
> +{
> +	int tmp_nr = nr_allowed_versions;
> +	enum protocol_version *tmp_allowed_versions, config_version;
> +	strbuf_reset(advert);
> +
> +	have_advertised_versions_already = 1;
> +
> +	config_version = get_protocol_version_config();
> +	if (config_version == protocol_v0) {
> +		strbuf_addstr(advert, "version=0");
> +		return;
> +	}
> +
> +	if (tmp_nr > 0) {
> +		ALLOC_ARRAY(tmp_allowed_versions, tmp_nr);
> +		copy_array(tmp_allowed_versions, allowed_versions, tmp_nr,
> +			   sizeof(enum protocol_version));
> +	} else {
> +		ALLOC_ARRAY(tmp_allowed_versions, 1);
> +		tmp_nr = 1;
> +		tmp_allowed_versions[0] = config_version;
> +	}
> +
> +	if (tmp_allowed_versions[0] != config_version)
> +		for (int i = 1; i < nr_allowed_versions; i++)
> +			if (tmp_allowed_versions[i] == config_version) {
> +				enum protocol_version swap =
> +					tmp_allowed_versions[0];
> +				tmp_allowed_versions[0] =
> +					tmp_allowed_versions[i];
> +				tmp_allowed_versions[i] = swap;
> +			}
> +
> +	strbuf_addf(advert, "version=%s",
> +		    format_protocol_version(tmp_allowed_versions[0]));
> +	for (int i = 1; i < tmp_nr; i++)
> +		strbuf_addf(advert, ":version=%s",
> +			    format_protocol_version(tmp_allowed_versions[i]));
> +}
> +

So the idea is that the protocols the other end can talk come in
advert in their preferred order, and we take an intersection of them
and our "allowed-versions", but the preference is further skewed
with the "swap" thing if we have our own preference specified via
config?

I am wondering if the code added by this patch outside this
function, with if (strcmp(client_ad.buf, "version=0") sprinkled all
over the place, works sensibly when the other side says "I prefer
version=0 but I do not mind talking version=1".

Isn't tmp_allowed_versions[] leaking when we return from this
function?

Re: [PATCH v3 1/1] protocol: advertise multiple supported versions

[Josh Steadmon]

On 2018.11.13 13:01, Junio C Hamano wrote:
> steadmon@google.com writes:
> 
> > Currently the client advertises that it supports the wire protocol
> > version set in the protocol.version config. However, not all services
> > support the same set of protocol versions. When connecting to
> > git-receive-pack, the client automatically downgrades to v0 if
> > config.protocol is set to v2, but this check is not performed for other
> > services.
> 
> "downgrades to v0 even if ... is set to v2" you mean?  Otherwise it
> is unclear why asking for v2 leads to using v0.

The downgrade on push happens only when the the configured version is
v2. If v1 is requested, no downgrade is triggered. I'll clarify the
commit message in the next version.

> > This patch creates a protocol version registry. Individual operations
> > register all the protocol versions they support prior to communicating
> > with a server. Versions should be listed in preference order; the
> > version specified in protocol.version will automatically be moved to the
> > front of the registry.
> >
> > The protocol version registry is passed to remote helpers via the
> > GIT_PROTOCOL environment variable.
> >
> > Clients now advertise the full list of registered versions. Servers
> > select the first recognized version from this advertisement.
> 
> Makes sense.
> 
> > +void get_client_protocol_version_advertisement(struct strbuf *advert)
> > +{
> > +	int tmp_nr = nr_allowed_versions;
> > +	enum protocol_version *tmp_allowed_versions, config_version;
> > +	strbuf_reset(advert);
> > +
> > +	have_advertised_versions_already = 1;
> > +
> > +	config_version = get_protocol_version_config();
> > +	if (config_version == protocol_v0) {
> > +		strbuf_addstr(advert, "version=0");
> > +		return;
> > +	}
> > +
> > +	if (tmp_nr > 0) {
> > +		ALLOC_ARRAY(tmp_allowed_versions, tmp_nr);
> > +		copy_array(tmp_allowed_versions, allowed_versions, tmp_nr,
> > +			   sizeof(enum protocol_version));
> > +	} else {
> > +		ALLOC_ARRAY(tmp_allowed_versions, 1);
> > +		tmp_nr = 1;
> > +		tmp_allowed_versions[0] = config_version;
> > +	}
> > +
> > +	if (tmp_allowed_versions[0] != config_version)
> > +		for (int i = 1; i < nr_allowed_versions; i++)
> > +			if (tmp_allowed_versions[i] == config_version) {
> > +				enum protocol_version swap =
> > +					tmp_allowed_versions[0];
> > +				tmp_allowed_versions[0] =
> > +					tmp_allowed_versions[i];
> > +				tmp_allowed_versions[i] = swap;
> > +			}
> > +
> > +	strbuf_addf(advert, "version=%s",
> > +		    format_protocol_version(tmp_allowed_versions[0]));
> > +	for (int i = 1; i < tmp_nr; i++)
> > +		strbuf_addf(advert, ":version=%s",
> > +			    format_protocol_version(tmp_allowed_versions[i]));
> > +}
> > +
> 
> So the idea is that the protocols the other end can talk come in
> advert in their preferred order, and we take an intersection of them
> and our "allowed-versions", but the preference is further skewed
> with the "swap" thing if we have our own preference specified via
> config?

We currently don't intersect with our own allowed list, we just accept
the first version that we recognize. This introduces its own version
negotiation bug; if we add v2 push in the future, a new client
talking to an old server would try to use v2 even though the server may
not have the corresponding v2 push implementation. I'll fix this in the
next version.

In any case, the ordering of the server's allowed versions won't matter;
we'll pick the the first version in the client's list which is also
allowed on the server.

> I am wondering if the code added by this patch outside this
> function, with if (strcmp(client_ad.buf, "version=0") sprinkled all
> over the place, works sensibly when the other side says "I prefer
> version=0 but I do not mind talking version=1".

Depends on what you mean by "sensibly" :). In the current case, if the
client prefers v0, it will always end up using v0. After the fixes
described above, it will always use v0 unless the server no longer
supports v0. Is that what you would expect?

> Isn't tmp_allowed_versions[] leaking when we return from this
> function?

Yes, sorry about that. Will fix.

Re: [PATCH v3 1/1] protocol: advertise multiple supported versions

[Junio C Hamano]

Josh Steadmon <steadmon@google.com> writes:

> On 2018.11.13 13:01, Junio C Hamano wrote:
>> steadmon@google.com writes:
>> 
>> > Currently the client advertises that it supports the wire protocol
>> > version set in the protocol.version config. However, not all services
>> > support the same set of protocol versions. When connecting to
>> > git-receive-pack, the client automatically downgrades to v0 if
>> > config.protocol is set to v2, but this check is not performed for other
>> > services.
>> 
>> "downgrades to v0 even if ... is set to v2" you mean?  Otherwise it
>> is unclear why asking for v2 leads to using v0.
>
> The downgrade on push happens only when the the configured version is
> v2. If v1 is requested, no downgrade is triggered. I'll clarify the
> commit message in the next version.

OK, then it will still be confusing unless "we downgrade v2 to v0
because ..."gives the reason.

> In any case, the ordering of the server's allowed versions won't matter;
> we'll pick the the first version in the client's list which is also
> allowed on the server.

That sounds like a very sensible semantics.

>
>> I am wondering if the code added by this patch outside this
>> function, with if (strcmp(client_ad.buf, "version=0") sprinkled all
>> over the place, works sensibly when the other side says "I prefer
>> version=0 but I do not mind talking version=1".
>
> Depends on what you mean by "sensibly" :). In the current case, if the
> client prefers v0, it will always end up using v0. After the fixes
> described above, it will always use v0 unless the server no longer
> supports v0. Is that what you would expect?

Yes, that sounds like a sensible behaviour.

What I was alludding to was a lot simpler, though.  An advert string
"version=0:version=1" from a client that prefers version 0 won't be
!strcmp("version=0", advert) but seeing many strcmp() in the patch
made me wonder.

Re: [PATCH v3 1/1] protocol: advertise multiple supported versions

[Josh Steadmon]

On 2018.11.14 11:38, Junio C Hamano wrote:
> Josh Steadmon <steadmon@google.com> writes:
> 
> > On 2018.11.13 13:01, Junio C Hamano wrote:
> >> I am wondering if the code added by this patch outside this
> >> function, with if (strcmp(client_ad.buf, "version=0") sprinkled all
> >> over the place, works sensibly when the other side says "I prefer
> >> version=0 but I do not mind talking version=1".
> >
> > Depends on what you mean by "sensibly" :). In the current case, if the
> > client prefers v0, it will always end up using v0. After the fixes
> > described above, it will always use v0 unless the server no longer
> > supports v0. Is that what you would expect?
> 
> Yes, that sounds like a sensible behaviour.
> 
> What I was alludding to was a lot simpler, though.  An advert string
> "version=0:version=1" from a client that prefers version 0 won't be
> !strcmp("version=0", advert) but seeing many strcmp() in the patch
> made me wonder.

Ah I see. The strcmp()s against "version=0" are special cases for where
it looks like the client does not understand any sort of version
negotiation. If we see multiple versions listed in the advert, then the
rest of the selection logic should do the right thing.

However, I think that it might work to remove the special cases. In the
event that the client is so old that it doesn't understand any form of
version negotiation, it should just ignore the version fields /
environment vars. If you think it's cleaner to remove the special cases,
let me know.

Re: [PATCH v3 1/1] protocol: advertise multiple supported versions

[Junio C Hamano]

Josh Steadmon <steadmon@google.com> writes:

>> What I was alludding to was a lot simpler, though.  An advert string
>> "version=0:version=1" from a client that prefers version 0 won't be
>> !strcmp("version=0", advert) but seeing many strcmp() in the patch
>> made me wonder.
>
> Ah I see. The strcmp()s against "version=0" are special cases for where
> it looks like the client does not understand any sort of version
> negotiation. If we see multiple versions listed in the advert, then the
> rest of the selection logic should do the right thing.

OK, let me try to see if I understand correctly by rephrasing.

If the client does not say anything about which version it prefers
(because it only knows about version 0 without even realizing that
there is a version negotiation exchange), we substitute the lack of
proposed versions with string "version=0", and the strcmp()s I saw
and was puzzled by are all about special casing such a client.  But
we would end up behaving the same way (at least when judged only by
externally visible effects) to a client that is aware of version
negotiation and tells us it prefers version 0 (and nothing else)
with the selection logic anyway.

Did I get it right?  If so, yeah, I think it makes sense to avoid
two codepaths that ends up doing the same thing by removing the
special case.

> However, I think that it might work to remove the special cases. In the
> event that the client is so old that it doesn't understand any form of
> version negotiation, it should just ignore the version fields /
> environment vars. If you think it's cleaner to remove the special cases,
> let me know.

Re: [PATCH v3 1/1] protocol: advertise multiple supported versions

[Josh Steadmon]

On 2018.11.16 11:45, Junio C Hamano wrote:
> Josh Steadmon <steadmon@google.com> writes:
> 
> >> What I was alludding to was a lot simpler, though.  An advert string
> >> "version=0:version=1" from a client that prefers version 0 won't be
> >> !strcmp("version=0", advert) but seeing many strcmp() in the patch
> >> made me wonder.
> >
> > Ah I see. The strcmp()s against "version=0" are special cases for where
> > it looks like the client does not understand any sort of version
> > negotiation. If we see multiple versions listed in the advert, then the
> > rest of the selection logic should do the right thing.
> 
> OK, let me try to see if I understand correctly by rephrasing.
> 
> If the client does not say anything about which version it prefers
> (because it only knows about version 0 without even realizing that
> there is a version negotiation exchange), we substitute the lack of
> proposed versions with string "version=0", and the strcmp()s I saw
> and was puzzled by are all about special casing such a client.  But
> we would end up behaving the same way (at least when judged only by
> externally visible effects) to a client that is aware of version
> negotiation and tells us it prefers version 0 (and nothing else)
> with the selection logic anyway.
> 
> Did I get it right?  If so, yeah, I think it makes sense to avoid
> two codepaths that ends up doing the same thing by removing the
> special case.

Yes, that is correct. The next version will remove the special cases
here.

> > However, I think that it might work to remove the special cases. In the
> > event that the client is so old that it doesn't understand any form of
> > version negotiation, it should just ignore the version fields /
> > environment vars. If you think it's cleaner to remove the special cases,
> > let me know.

Re: [PATCH v3 1/1] protocol: advertise multiple supported versions

[Junio C Hamano]

steadmon@google.com writes:

> +	if (tmp_allowed_versions[0] != config_version)
> +		for (int i = 1; i < nr_allowed_versions; i++)
> +			if (tmp_allowed_versions[i] == config_version) {
> +				enum protocol_version swap =
> +					tmp_allowed_versions[0];
> +				tmp_allowed_versions[0] =
> +					tmp_allowed_versions[i];
> +				tmp_allowed_versions[i] = swap;
> +			}

Here is what coccicheck suggests.

diff -u -p a/protocol.c b/protocol.c
--- a/protocol.c
+++ b/protocol.c
@@ -111,11 +111,8 @@ void get_client_protocol_version_adverti
 	if (tmp_allowed_versions[0] != config_version)
 		for (int i = 1; i < nr_allowed_versions; i++)
 			if (tmp_allowed_versions[i] == config_version) {
-				enum protocol_version swap =
-					tmp_allowed_versions[0];
-				tmp_allowed_versions[0] =
-					tmp_allowed_versions[i];
-				tmp_allowed_versions[i] = swap;
+				SWAP(tmp_allowed_versions[0],
+				     tmp_allowed_versions[i]);
 			}
 
 	strbuf_addf(advert, "version=%s",

Re: [PATCH v3 1/1] protocol: advertise multiple supported versions

[SZEDER Gábor]

On Mon, Nov 12, 2018 at 01:49:05PM -0800, steadmon@google.com wrote:

> diff --git a/protocol.c b/protocol.c
> index 5e636785d1..54d2ab991b 100644
> --- a/protocol.c
> +++ b/protocol.c

> +void get_client_protocol_version_advertisement(struct strbuf *advert)
> +{
> +	int tmp_nr = nr_allowed_versions;
> +	enum protocol_version *tmp_allowed_versions, config_version;
> +	strbuf_reset(advert);
> +
> +	have_advertised_versions_already = 1;
> +
> +	config_version = get_protocol_version_config();
> +	if (config_version == protocol_v0) {
> +		strbuf_addstr(advert, "version=0");
> +		return;
> +	}
> +
> +	if (tmp_nr > 0) {
> +		ALLOC_ARRAY(tmp_allowed_versions, tmp_nr);
> +		copy_array(tmp_allowed_versions, allowed_versions, tmp_nr,
> +			   sizeof(enum protocol_version));
> +	} else {
> +		ALLOC_ARRAY(tmp_allowed_versions, 1);
> +		tmp_nr = 1;
> +		tmp_allowed_versions[0] = config_version;
> +	}
> +
> +	if (tmp_allowed_versions[0] != config_version)
> +		for (int i = 1; i < nr_allowed_versions; i++)

We don't do C99 yet, thus the declaration of a loop variable like this
is not allowed and triggers compiler errors.

> +			if (tmp_allowed_versions[i] == config_version) {
> +				enum protocol_version swap =
> +					tmp_allowed_versions[0];
> +				tmp_allowed_versions[0] =
> +					tmp_allowed_versions[i];
> +				tmp_allowed_versions[i] = swap;
> +			}
> +
> +	strbuf_addf(advert, "version=%s",
> +		    format_protocol_version(tmp_allowed_versions[0]));
> +	for (int i = 1; i < tmp_nr; i++)

Likewise.

> +		strbuf_addf(advert, ":version=%s",
> +			    format_protocol_version(tmp_allowed_versions[i]));
> +}

Re: [PATCH v3 1/1] protocol: advertise multiple supported versions

[Josh Steadmon]

On 2018.11.13 19:28, SZEDER Gábor wrote:
> On Mon, Nov 12, 2018 at 01:49:05PM -0800, steadmon@google.com wrote:
> 
> > diff --git a/protocol.c b/protocol.c
> > index 5e636785d1..54d2ab991b 100644
> > --- a/protocol.c
> > +++ b/protocol.c
> 
> > +void get_client_protocol_version_advertisement(struct strbuf *advert)
> > +{
> > +	int tmp_nr = nr_allowed_versions;
> > +	enum protocol_version *tmp_allowed_versions, config_version;
> > +	strbuf_reset(advert);
> > +
> > +	have_advertised_versions_already = 1;
> > +
> > +	config_version = get_protocol_version_config();
> > +	if (config_version == protocol_v0) {
> > +		strbuf_addstr(advert, "version=0");
> > +		return;
> > +	}
> > +
> > +	if (tmp_nr > 0) {
> > +		ALLOC_ARRAY(tmp_allowed_versions, tmp_nr);
> > +		copy_array(tmp_allowed_versions, allowed_versions, tmp_nr,
> > +			   sizeof(enum protocol_version));
> > +	} else {
> > +		ALLOC_ARRAY(tmp_allowed_versions, 1);
> > +		tmp_nr = 1;
> > +		tmp_allowed_versions[0] = config_version;
> > +	}
> > +
> > +	if (tmp_allowed_versions[0] != config_version)
> > +		for (int i = 1; i < nr_allowed_versions; i++)
> 
> We don't do C99 yet, thus the declaration of a loop variable like this
> is not allowed and triggers compiler errors.
> 
> > +			if (tmp_allowed_versions[i] == config_version) {
> > +				enum protocol_version swap =
> > +					tmp_allowed_versions[0];
> > +				tmp_allowed_versions[0] =
> > +					tmp_allowed_versions[i];
> > +				tmp_allowed_versions[i] = swap;
> > +			}
> > +
> > +	strbuf_addf(advert, "version=%s",
> > +		    format_protocol_version(tmp_allowed_versions[0]));
> > +	for (int i = 1; i < tmp_nr; i++)
> 
> Likewise.
> 
> > +		strbuf_addf(advert, ":version=%s",
> > +			    format_protocol_version(tmp_allowed_versions[i]));
> > +}

Sorry about that. Will fix in v4. Out of curiousity, do you have a
config.mak snippet that will make these into errors? I played around
with adding combinations of -ansi, -std=c89, and -pedantic to CFLAGS,
but I couldn't get anything that detect the problem without also
breaking on other parts of the build.

Re: [PATCH v3 1/1] protocol: advertise multiple supported versions

[SZEDER Gábor]

On Tue, Nov 13, 2018 at 03:03:47PM -0800, Josh Steadmon wrote:
> > > +		for (int i = 1; i < nr_allowed_versions; i++)
> > 
> > We don't do C99 yet, thus the declaration of a loop variable like this
> > is not allowed and triggers compiler errors.

> Sorry about that. Will fix in v4. Out of curiousity, do you have a
> config.mak snippet that will make these into errors? I played around
> with adding combinations of -ansi, -std=c89, and -pedantic to CFLAGS,
> but I couldn't get anything that detect the problem without also
> breaking on other parts of the build.

Unfortunately, I don't have such an universal CFLAGS.

With gcc-4.8 the default CFLAGS is sufficient:

  $ make V=1 CC=gcc-4.8 protocol.o
  gcc-4.8 -o protocol.o -c -MF ./.depend/protocol.o.d -MQ protocol.o -MMD -MP  -g -O2 -Wall -I. -DHAVE_SYSINFO -DGIT_HOST_CPU="\"x86_64\"" -DHAVE_ALLOCA_H -DUSE_CURL_FOR_IMAP_SEND -DSHA1_DC -DSHA1DC_NO_STANDARD_INCLUDES -DSHA1DC_INIT_SAFE_HASH_DEFAULT=0 -DSHA1DC_CUSTOM_INCLUDE_SHA1_C="\"cache.h\"" -DSHA1DC_CUSTOM_INCLUDE_UBC_CHECK_C="\"git-compat-util.h\""  -DHAVE_PATHS_H -DHAVE_DEV_TTY -DHAVE_CLOCK_GETTIME -DHAVE_CLOCK_MONOTONIC -DHAVE_GETDELIM '-DPROCFS_EXECUTABLE_PATH="/proc/self/exe"'  -DFREAD_READS_DIRECTORIES -DNO_STRLCPY -DSHELL_PATH='"/bin/sh"' -DPAGER_ENV='"LESS=FRX LV=-c"'  protocol.c
  protocol.c: In function ‘get_client_protocol_version_advertisement’:
  protocol.c:112:3: error: ‘for’ loop initial declarations are only allowed in C99 mode
     for (int i = 1; i < nr_allowed_versions; i++)
     ^
  < ... snip ... >

I couldn't get this error with any newer GCC or Clang, and using
options like -std=c89 trigger many other errors as well, just like you
mentioned.

Re: [PATCH v3 1/1] protocol: advertise multiple supported versions

[Junio C Hamano]

SZEDER Gábor <szeder.dev@gmail.com> writes:

>> +	if (tmp_allowed_versions[0] != config_version)
>> +		for (int i = 1; i < nr_allowed_versions; i++)
>
> We don't do C99 yet, thus the declaration of a loop variable like this
> is not allowed and triggers compiler errors.

I thought we did a weather-balloon to see if this bothers people who
build on minority platforms but

	git grep 'for (int'

is coming up empty.

We have been trying designated initializers with weather-balloon
changes (both arrays and struct fields) and I somehow thought that
we already were trying this out, but apparently that is not the
case.  Also trailing comma in the enum definition was what we
discussed earlier but I do not know if we had weather-balloon for it
or not offhand.  

Perhaps we should write these down in CodingGuidelines.

Re: [PATCH v3 1/1] protocol: advertise multiple supported versions

[SZEDER Gábor]

On Wed, Nov 14, 2018 at 11:44:51AM +0900, Junio C Hamano wrote:
> SZEDER Gábor <szeder.dev@gmail.com> writes:
> 
> >> +	if (tmp_allowed_versions[0] != config_version)
> >> +		for (int i = 1; i < nr_allowed_versions; i++)
> >
> > We don't do C99 yet, thus the declaration of a loop variable like this
> > is not allowed and triggers compiler errors.
> 
> I thought we did a weather-balloon to see if this bothers people who
> build on minority platforms but
> 
> 	git grep 'for (int'
> 
> is coming up empty.
> 
> We have been trying designated initializers with weather-balloon
> changes (both arrays and struct fields) and I somehow thought that
> we already were trying this out, but apparently that is not the
> case.

I thought so as well, and I run the exact same 'git grep' command
before replying to Josh :)

There was a discussion about such a weather-balloon patch [1] (which
happened to use an unsigned loop variable, so our grep wouldn't have
found it anyway), but it wasn't picked up because it required new
options in CFLAGS and there was no standard way to do so [2].

[1] https://public-inbox.org/git/20170719181956.15845-1-sbeller@google.com/
[2] https://public-inbox.org/git/20170724170813.scceigybl5d3fvdd@sigill.intra.peff.net/

[PATCH v4 0/1] Advertise multiple supported proto versions

[Josh Steadmon]

Fix several bugs identified in v3, clarify commit message, and clean up
extern keyword in protocol.h.

Josh Steadmon (1):
  protocol: advertise multiple supported versions

 builtin/archive.c        |   3 +
 builtin/clone.c          |   4 ++
 builtin/fetch-pack.c     |   4 ++
 builtin/fetch.c          |   5 ++
 builtin/ls-remote.c      |   5 ++
 builtin/pull.c           |   5 ++
 builtin/push.c           |   4 ++
 builtin/receive-pack.c   |   3 +
 builtin/send-pack.c      |   3 +
 builtin/upload-archive.c |   3 +
 builtin/upload-pack.c    |   4 ++
 connect.c                |  47 +++++++--------
 protocol.c               | 122 ++++++++++++++++++++++++++++++++++++---
 protocol.h               |  23 +++++++-
 remote-curl.c            |  28 ++++++---
 t/t5570-git-daemon.sh    |   2 +-
 t/t5700-protocol-v1.sh   |   8 +--
 t/t5702-protocol-v2.sh   |  16 +++--
 transport-helper.c       |   6 ++
 19 files changed, 237 insertions(+), 58 deletions(-)

Range-diff against v3:
1:  b9968e3fb0 ! 1:  3c023991c0 protocol: advertise multiple supported versions
    @@ -4,22 +4,25 @@
     
         Currently the client advertises that it supports the wire protocol
         version set in the protocol.version config. However, not all services
    -    support the same set of protocol versions. When connecting to
    -    git-receive-pack, the client automatically downgrades to v0 if
    -    config.protocol is set to v2, but this check is not performed for other
    -    services.
    +    support the same set of protocol versions. For example, git-receive-pack
    +    supports v1 and v0, but not v2. If a client connects to git-receive-pack
    +    and requests v2, it will instead be downgraded to v0. Other services,
    +    such as git-upload-archive, do not do any version negotiation checks.
     
    -    This patch creates a protocol version registry. Individual operations
    -    register all the protocol versions they support prior to communicating
    -    with a server. Versions should be listed in preference order; the
    -    version specified in protocol.version will automatically be moved to the
    -    front of the registry.
    +    This patch creates a protocol version registry. Individual client and
    +    server programs register all the protocol versions they support prior to
    +    communicating with a remote instance. Versions should be listed in
    +    preference order; the version specified in protocol.version will
    +    automatically be moved to the front of the registry.
     
         The protocol version registry is passed to remote helpers via the
         GIT_PROTOCOL environment variable.
     
         Clients now advertise the full list of registered versions. Servers
    -    select the first recognized version from this advertisement.
    +    select the first allowed version from this advertisement.
    +
    +    While we're at it, remove unnecessary externs from function declarations
    +    in protocol.h.
     
         Signed-off-by: Josh Steadmon <steadmon@google.com>
    @@ -165,6 +168,20 @@
      	git_config(git_push_config, &flags);
      	argc = parse_options(argc, argv, prefix, options, push_usage, 0);
     
    + diff --git a/builtin/receive-pack.c b/builtin/receive-pack.c
    + --- a/builtin/receive-pack.c
    + +++ b/builtin/receive-pack.c
    +@@
    + 
    + 	packet_trace_identity("receive-pack");
    + 
    ++	register_allowed_protocol_version(protocol_v1);
    ++	register_allowed_protocol_version(protocol_v0);
    ++
    + 	argc = parse_options(argc, argv, prefix, options, receive_pack_usage, 0);
    + 
    + 	if (argc > 1)
    +
      diff --git a/builtin/send-pack.c b/builtin/send-pack.c
      --- a/builtin/send-pack.c
      +++ b/builtin/send-pack.c
    @@ -179,6 +196,42 @@
      	argc = parse_options(argc, argv, prefix, options, send_pack_usage, 0);
      	if (argc > 0) {
     
    + diff --git a/builtin/upload-archive.c b/builtin/upload-archive.c
    + --- a/builtin/upload-archive.c
    + +++ b/builtin/upload-archive.c
    +@@
    + #include "builtin.h"
    + #include "archive.h"
    + #include "pkt-line.h"
    ++#include "protocol.h"
    + #include "sideband.h"
    + #include "run-command.h"
    + #include "argv-array.h"
    +@@
    + 	if (argc == 2 && !strcmp(argv[1], "-h"))
    + 		usage(upload_archive_usage);
    + 
    ++	register_allowed_protocol_version(protocol_v0);
    ++
    + 	/*
    + 	 * Set up sideband subprocess.
    + 	 *
    +
    + diff --git a/builtin/upload-pack.c b/builtin/upload-pack.c
    + --- a/builtin/upload-pack.c
    + +++ b/builtin/upload-pack.c
    +@@
    + 	packet_trace_identity("upload-pack");
    + 	read_replace_refs = 0;
    + 
    ++	register_allowed_protocol_version(protocol_v2);
    ++	register_allowed_protocol_version(protocol_v1);
    ++	register_allowed_protocol_version(protocol_v0);
    ++
    + 	argc = parse_options(argc, argv, NULL, options, upload_pack_usage, 0);
    + 
    + 	if (argc != 1)
    +
      diff --git a/connect.c b/connect.c
      --- a/connect.c
      +++ b/connect.c
    @@ -311,7 +364,7 @@
     +static enum protocol_version *allowed_versions;
     +static int nr_allowed_versions;
     +static int alloc_allowed_versions;
    -+static int have_advertised_versions_already = 0;
    ++static int version_registration_locked = 0;
     +
     +static const char protocol_v0_string[] = "0";
     +static const char protocol_v1_string[] = "1";
    @@ -357,8 +410,8 @@
      
     +void register_allowed_protocol_version(enum protocol_version version)
     +{
    -+	if (have_advertised_versions_already)
    -+		BUG(_("attempting to register an allowed protocol version after advertisement"));
    ++	if (version_registration_locked)
    ++		BUG("late attempt to register an allowed protocol version");
     +
     +	ALLOC_GROW(allowed_versions, nr_allowed_versions + 1,
     +		   alloc_allowed_versions);
    @@ -384,13 +437,23 @@
     +	string_list_clear(&version_list, 0);
     +}
     +
    ++static int is_allowed_protocol_version(enum protocol_version version)
    ++{
    ++	int i;
    ++	version_registration_locked = 1;
    ++	for (i = 0; i < nr_allowed_versions; i++)
    ++		if (version == allowed_versions[i])
    ++			return 1;
    ++	return 0;
    ++}
    ++
     +void get_client_protocol_version_advertisement(struct strbuf *advert)
     +{
    -+	int tmp_nr = nr_allowed_versions;
    ++	int i, tmp_nr = nr_allowed_versions;
     +	enum protocol_version *tmp_allowed_versions, config_version;
     +	strbuf_reset(advert);
     +
    -+	have_advertised_versions_already = 1;
    ++	version_registration_locked = 1;
     +
     +	config_version = get_protocol_version_config();
     +	if (config_version == protocol_v0) {
    @@ -409,20 +472,19 @@
     +	}
     +
     +	if (tmp_allowed_versions[0] != config_version)
    -+		for (int i = 1; i < nr_allowed_versions; i++)
    ++		for (i = 1; i < nr_allowed_versions; i++)
     +			if (tmp_allowed_versions[i] == config_version) {
    -+				enum protocol_version swap =
    -+					tmp_allowed_versions[0];
    -+				tmp_allowed_versions[0] =
    -+					tmp_allowed_versions[i];
    -+				tmp_allowed_versions[i] = swap;
    ++				SWAP(tmp_allowed_versions[0],
    ++				     tmp_allowed_versions[i]);
     +			}
     +
     +	strbuf_addf(advert, "version=%s",
     +		    format_protocol_version(tmp_allowed_versions[0]));
    -+	for (int i = 1; i < tmp_nr; i++)
    ++	for (i = 1; i < tmp_nr; i++)
     +		strbuf_addf(advert, ":version=%s",
     +			    format_protocol_version(tmp_allowed_versions[i]));
    ++
    ++	free(tmp_allowed_versions);
     +}
     +
      enum protocol_version determine_protocol_version_server(void)
    @@ -447,7 +509,8 @@
      			if (skip_prefix(item->string, "version=", &value)) {
      				v = parse_protocol_version(value);
     -				if (v > version)
    -+				if (v != protocol_unknown_version) {
    ++				if (v != protocol_unknown_version &&
    ++				    is_allowed_protocol_version(v)) {
      					version = v;
     +					break;
     +				}
    @@ -459,29 +522,46 @@
      --- a/protocol.h
      +++ b/protocol.h
     @@
    +  * 'protocol.version' config.  If unconfigured, a value of 'protocol_v0' is
    +  * returned.
       */
    - extern enum protocol_version get_protocol_version_config(void);
    - 
    +-extern enum protocol_version get_protocol_version_config(void);
    ++enum protocol_version get_protocol_version_config(void);
    ++
     +/*
     + * Register an allowable protocol version for a given operation. Registration
     + * must occur before attempting to advertise a version to a server process.
     + */
    -+extern void register_allowed_protocol_version(enum protocol_version version);
    ++void register_allowed_protocol_version(enum protocol_version version);
     +
     +/*
     + * Register allowable protocol versions from the GIT_PROTOCOL environment var.
     + */
    -+extern void register_allowed_protocol_versions_from_env(void);
    ++void register_allowed_protocol_versions_from_env(void);
     +
     +/*
     + * Fill a strbuf with a version advertisement string suitable for use in the
     + * GIT_PROTOCOL environment variable or similar version negotiation field.
     + */
    -+extern void get_client_protocol_version_advertisement(struct strbuf *advert);
    -+
    ++void get_client_protocol_version_advertisement(struct strbuf *advert);
    + 
      /*
       * Used by a server to determine which protocol version should be used based on
    -  * a client's request, communicated via the 'GIT_PROTOCOL' environment variable
    +@@
    +  * request a particular protocol version, a default of 'protocol_v0' will be
    +  * used.
    +  */
    +-extern enum protocol_version determine_protocol_version_server(void);
    ++enum protocol_version determine_protocol_version_server(void);
    + 
    + /*
    +  * Used by a client to determine which protocol version the server is speaking
    +  * based on the server's initial response.
    +  */
    +-extern enum protocol_version determine_protocol_version_client(const char *server_response);
    ++enum protocol_version determine_protocol_version_client(const char *server_response);
    + 
    + #endif /* PROTOCOL_H */
     
      diff --git a/remote-curl.c b/remote-curl.c
      --- a/remote-curl.c
-- 
2.19.1.930.g4563a0d9d0-goog

[PATCH v4 1/1] protocol: advertise multiple supported versions

[Josh Steadmon]

Currently the client advertises that it supports the wire protocol
version set in the protocol.version config. However, not all services
support the same set of protocol versions. For example, git-receive-pack
supports v1 and v0, but not v2. If a client connects to git-receive-pack
and requests v2, it will instead be downgraded to v0. Other services,
such as git-upload-archive, do not do any version negotiation checks.

This patch creates a protocol version registry. Individual client and
server programs register all the protocol versions they support prior to
communicating with a remote instance. Versions should be listed in
preference order; the version specified in protocol.version will
automatically be moved to the front of the registry.

The protocol version registry is passed to remote helpers via the
GIT_PROTOCOL environment variable.

Clients now advertise the full list of registered versions. Servers
select the first allowed version from this advertisement.

While we're at it, remove unnecessary externs from function declarations
in protocol.h.

Signed-off-by: Josh Steadmon <steadmon@google.com>
---
 builtin/archive.c        |   3 +
 builtin/clone.c          |   4 ++
 builtin/fetch-pack.c     |   4 ++
 builtin/fetch.c          |   5 ++
 builtin/ls-remote.c      |   5 ++
 builtin/pull.c           |   5 ++
 builtin/push.c           |   4 ++
 builtin/receive-pack.c   |   3 +
 builtin/send-pack.c      |   3 +
 builtin/upload-archive.c |   3 +
 builtin/upload-pack.c    |   4 ++
 connect.c                |  47 +++++++--------
 protocol.c               | 122 ++++++++++++++++++++++++++++++++++++---
 protocol.h               |  23 +++++++-
 remote-curl.c            |  28 ++++++---
 t/t5570-git-daemon.sh    |   2 +-
 t/t5700-protocol-v1.sh   |   8 +--
 t/t5702-protocol-v2.sh   |  16 +++--
 transport-helper.c       |   6 ++
 19 files changed, 237 insertions(+), 58 deletions(-)

diff --git a/builtin/archive.c b/builtin/archive.c
index e74f675390..8adb9f381b 100644
--- a/builtin/archive.c
+++ b/builtin/archive.c
@@ -8,6 +8,7 @@
 #include "transport.h"
 #include "parse-options.h"
 #include "pkt-line.h"
+#include "protocol.h"
 #include "sideband.h"
 
 static void create_output_file(const char *output_file)
@@ -94,6 +95,8 @@ int cmd_archive(int argc, const char **argv, const char *prefix)
 		OPT_END()
 	};
 
+	register_allowed_protocol_version(protocol_v0);
+
 	argc = parse_options(argc, argv, prefix, local_opts, NULL,
 			     PARSE_OPT_KEEP_ALL);
 
diff --git a/builtin/clone.c b/builtin/clone.c
index fd2c3ef090..1651a950b6 100644
--- a/builtin/clone.c
+++ b/builtin/clone.c
@@ -900,6 +900,10 @@ int cmd_clone(int argc, const char **argv, const char *prefix)
 	struct refspec rs = REFSPEC_INIT_FETCH;
 	struct argv_array ref_prefixes = ARGV_ARRAY_INIT;
 
+	register_allowed_protocol_version(protocol_v2);
+	register_allowed_protocol_version(protocol_v1);
+	register_allowed_protocol_version(protocol_v0);
+
 	fetch_if_missing = 0;
 
 	packet_trace_identity("clone");
diff --git a/builtin/fetch-pack.c b/builtin/fetch-pack.c
index 1a1bc63566..cba935e4d3 100644
--- a/builtin/fetch-pack.c
+++ b/builtin/fetch-pack.c
@@ -57,6 +57,10 @@ int cmd_fetch_pack(int argc, const char **argv, const char *prefix)
 
 	fetch_if_missing = 0;
 
+	register_allowed_protocol_version(protocol_v2);
+	register_allowed_protocol_version(protocol_v1);
+	register_allowed_protocol_version(protocol_v0);
+
 	packet_trace_identity("fetch-pack");
 
 	memset(&args, 0, sizeof(args));
diff --git a/builtin/fetch.c b/builtin/fetch.c
index 61bec5d213..2a20cf8bfd 100644
--- a/builtin/fetch.c
+++ b/builtin/fetch.c
@@ -21,6 +21,7 @@
 #include "argv-array.h"
 #include "utf8.h"
 #include "packfile.h"
+#include "protocol.h"
 #include "list-objects-filter-options.h"
 
 static const char * const builtin_fetch_usage[] = {
@@ -1476,6 +1477,10 @@ int cmd_fetch(int argc, const char **argv, const char *prefix)
 	int prune_tags_ok = 1;
 	struct argv_array argv_gc_auto = ARGV_ARRAY_INIT;
 
+	register_allowed_protocol_version(protocol_v2);
+	register_allowed_protocol_version(protocol_v1);
+	register_allowed_protocol_version(protocol_v0);
+
 	packet_trace_identity("fetch");
 
 	fetch_if_missing = 0;
diff --git a/builtin/ls-remote.c b/builtin/ls-remote.c
index 1a25df7ee1..ea685e8bb9 100644
--- a/builtin/ls-remote.c
+++ b/builtin/ls-remote.c
@@ -1,5 +1,6 @@
 #include "builtin.h"
 #include "cache.h"
+#include "protocol.h"
 #include "transport.h"
 #include "ref-filter.h"
 #include "remote.h"
@@ -80,6 +81,10 @@ int cmd_ls_remote(int argc, const char **argv, const char *prefix)
 
 	memset(&ref_array, 0, sizeof(ref_array));
 
+	register_allowed_protocol_version(protocol_v2);
+	register_allowed_protocol_version(protocol_v1);
+	register_allowed_protocol_version(protocol_v0);
+
 	argc = parse_options(argc, argv, prefix, options, ls_remote_usage,
 			     PARSE_OPT_STOP_AT_NON_OPTION);
 	dest = argv[0];
diff --git a/builtin/pull.c b/builtin/pull.c
index 681c127a07..cb64146834 100644
--- a/builtin/pull.c
+++ b/builtin/pull.c
@@ -9,6 +9,7 @@
 #include "config.h"
 #include "builtin.h"
 #include "parse-options.h"
+#include "protocol.h"
 #include "exec-cmd.h"
 #include "run-command.h"
 #include "sha1-array.h"
@@ -849,6 +850,10 @@ int cmd_pull(int argc, const char **argv, const char *prefix)
 	struct object_id rebase_fork_point;
 	int autostash;
 
+	register_allowed_protocol_version(protocol_v2);
+	register_allowed_protocol_version(protocol_v1);
+	register_allowed_protocol_version(protocol_v0);
+
 	if (!getenv("GIT_REFLOG_ACTION"))
 		set_reflog_message(argc, argv);
 
diff --git a/builtin/push.c b/builtin/push.c
index d09a42062c..10d8abe829 100644
--- a/builtin/push.c
+++ b/builtin/push.c
@@ -10,6 +10,7 @@
 #include "remote.h"
 #include "transport.h"
 #include "parse-options.h"
+#include "protocol.h"
 #include "submodule.h"
 #include "submodule-config.h"
 #include "send-pack.h"
@@ -587,6 +588,9 @@ int cmd_push(int argc, const char **argv, const char *prefix)
 		OPT_END()
 	};
 
+	register_allowed_protocol_version(protocol_v1);
+	register_allowed_protocol_version(protocol_v0);
+
 	packet_trace_identity("push");
 	git_config(git_push_config, &flags);
 	argc = parse_options(argc, argv, prefix, options, push_usage, 0);
diff --git a/builtin/receive-pack.c b/builtin/receive-pack.c
index c17ce94e12..030cb7b7ec 100644
--- a/builtin/receive-pack.c
+++ b/builtin/receive-pack.c
@@ -1929,6 +1929,9 @@ int cmd_receive_pack(int argc, const char **argv, const char *prefix)
 
 	packet_trace_identity("receive-pack");
 
+	register_allowed_protocol_version(protocol_v1);
+	register_allowed_protocol_version(protocol_v0);
+
 	argc = parse_options(argc, argv, prefix, options, receive_pack_usage, 0);
 
 	if (argc > 1)
diff --git a/builtin/send-pack.c b/builtin/send-pack.c
index 8e3c7490f7..f48bd1306b 100644
--- a/builtin/send-pack.c
+++ b/builtin/send-pack.c
@@ -184,6 +184,9 @@ int cmd_send_pack(int argc, const char **argv, const char *prefix)
 		OPT_END()
 	};
 
+	register_allowed_protocol_version(protocol_v1);
+	register_allowed_protocol_version(protocol_v0);
+
 	git_config(send_pack_config, NULL);
 	argc = parse_options(argc, argv, prefix, options, send_pack_usage, 0);
 	if (argc > 0) {
diff --git a/builtin/upload-archive.c b/builtin/upload-archive.c
index 25d9116356..791cbe80a6 100644
--- a/builtin/upload-archive.c
+++ b/builtin/upload-archive.c
@@ -5,6 +5,7 @@
 #include "builtin.h"
 #include "archive.h"
 #include "pkt-line.h"
+#include "protocol.h"
 #include "sideband.h"
 #include "run-command.h"
 #include "argv-array.h"
@@ -80,6 +81,8 @@ int cmd_upload_archive(int argc, const char **argv, const char *prefix)
 	if (argc == 2 && !strcmp(argv[1], "-h"))
 		usage(upload_archive_usage);
 
+	register_allowed_protocol_version(protocol_v0);
+
 	/*
 	 * Set up sideband subprocess.
 	 *
diff --git a/builtin/upload-pack.c b/builtin/upload-pack.c
index 42dc4da5a1..293dd45b9e 100644
--- a/builtin/upload-pack.c
+++ b/builtin/upload-pack.c
@@ -33,6 +33,10 @@ int cmd_upload_pack(int argc, const char **argv, const char *prefix)
 	packet_trace_identity("upload-pack");
 	read_replace_refs = 0;
 
+	register_allowed_protocol_version(protocol_v2);
+	register_allowed_protocol_version(protocol_v1);
+	register_allowed_protocol_version(protocol_v0);
+
 	argc = parse_options(argc, argv, NULL, options, upload_pack_usage, 0);
 
 	if (argc != 1)
diff --git a/connect.c b/connect.c
index 94547e5056..a170ba1e90 100644
--- a/connect.c
+++ b/connect.c
@@ -1046,7 +1046,7 @@ static enum ssh_variant determine_ssh_variant(const char *ssh_command,
  */
 static struct child_process *git_connect_git(int fd[2], char *hostandport,
 					     const char *path, const char *prog,
-					     enum protocol_version version,
+					     const struct strbuf *version_advert,
 					     int flags)
 {
 	struct child_process *conn;
@@ -1085,10 +1085,9 @@ static struct child_process *git_connect_git(int fd[2], char *hostandport,
 		    target_host, 0);
 
 	/* If using a new version put that stuff here after a second null byte */
-	if (version > 0) {
+	if (strcmp(version_advert->buf, "version=0")) {
 		strbuf_addch(&request, '\0');
-		strbuf_addf(&request, "version=%d%c",
-			    version, '\0');
+		strbuf_addf(&request, "%s%c", version_advert->buf, '\0');
 	}
 
 	packet_write(fd[1], request.buf, request.len);
@@ -1104,14 +1103,14 @@ static struct child_process *git_connect_git(int fd[2], char *hostandport,
  */
 static void push_ssh_options(struct argv_array *args, struct argv_array *env,
 			     enum ssh_variant variant, const char *port,
-			     enum protocol_version version, int flags)
+			     const struct strbuf *version_advert, int flags)
 {
 	if (variant == VARIANT_SSH &&
-	    version > 0) {
+	    strcmp(version_advert->buf, "version=0")) {
 		argv_array_push(args, "-o");
 		argv_array_push(args, "SendEnv=" GIT_PROTOCOL_ENVIRONMENT);
-		argv_array_pushf(env, GIT_PROTOCOL_ENVIRONMENT "=version=%d",
-				 version);
+		argv_array_pushf(env, GIT_PROTOCOL_ENVIRONMENT "=%s",
+				 version_advert->buf);
 	}
 
 	if (flags & CONNECT_IPV4) {
@@ -1164,7 +1163,7 @@ static void push_ssh_options(struct argv_array *args, struct argv_array *env,
 
 /* Prepare a child_process for use by Git's SSH-tunneled transport. */
 static void fill_ssh_args(struct child_process *conn, const char *ssh_host,
-			  const char *port, enum protocol_version version,
+			  const char *port, const struct strbuf *version_advert,
 			  int flags)
 {
 	const char *ssh;
@@ -1198,15 +1197,16 @@ static void fill_ssh_args(struct child_process *conn, const char *ssh_host,
 
 		argv_array_push(&detect.args, ssh);
 		argv_array_push(&detect.args, "-G");
-		push_ssh_options(&detect.args, &detect.env_array,
-				 VARIANT_SSH, port, version, flags);
+		push_ssh_options(&detect.args, &detect.env_array, VARIANT_SSH,
+				 port, version_advert, flags);
 		argv_array_push(&detect.args, ssh_host);
 
 		variant = run_command(&detect) ? VARIANT_SIMPLE : VARIANT_SSH;
 	}
 
 	argv_array_push(&conn->args, ssh);
-	push_ssh_options(&conn->args, &conn->env_array, variant, port, version, flags);
+	push_ssh_options(&conn->args, &conn->env_array, variant, port,
+			 version_advert, flags);
 	argv_array_push(&conn->args, ssh_host);
 }
 
@@ -1226,16 +1226,10 @@ struct child_process *git_connect(int fd[2], const char *url,
 {
 	char *hostandport, *path;
 	struct child_process *conn;
+	struct strbuf version_advert = STRBUF_INIT;
 	enum protocol protocol;
-	enum protocol_version version = get_protocol_version_config();
 
-	/*
-	 * NEEDSWORK: If we are trying to use protocol v2 and we are planning
-	 * to perform a push, then fallback to v0 since the client doesn't know
-	 * how to push yet using v2.
-	 */
-	if (version == protocol_v2 && !strcmp("git-receive-pack", prog))
-		version = protocol_v0;
+	get_client_protocol_version_advertisement(&version_advert);
 
 	/* Without this we cannot rely on waitpid() to tell
 	 * what happened to our children.
@@ -1250,7 +1244,8 @@ struct child_process *git_connect(int fd[2], const char *url,
 		printf("Diag: path=%s\n", path ? path : "NULL");
 		conn = NULL;
 	} else if (protocol == PROTO_GIT) {
-		conn = git_connect_git(fd, hostandport, path, prog, version, flags);
+		conn = git_connect_git(fd, hostandport, path, prog,
+				       &version_advert, flags);
 	} else {
 		struct strbuf cmd = STRBUF_INIT;
 		const char *const *var;
@@ -1293,12 +1288,14 @@ struct child_process *git_connect(int fd[2], const char *url,
 				strbuf_release(&cmd);
 				return NULL;
 			}
-			fill_ssh_args(conn, ssh_host, port, version, flags);
+			fill_ssh_args(conn, ssh_host, port, &version_advert,
+				      flags);
 		} else {
 			transport_check_allowed("file");
-			if (version > 0) {
-				argv_array_pushf(&conn->env_array, GIT_PROTOCOL_ENVIRONMENT "=version=%d",
-						 version);
+			if (strcmp(version_advert.buf, "version=0")) {
+				argv_array_pushf(&conn->env_array,
+						 GIT_PROTOCOL_ENVIRONMENT "=%s",
+						 version_advert.buf);
 			}
 		}
 		argv_array_push(&conn->args, cmd.buf);
diff --git a/protocol.c b/protocol.c
index 5e636785d1..5664bd7a05 100644
--- a/protocol.c
+++ b/protocol.c
@@ -2,18 +2,43 @@
 #include "config.h"
 #include "protocol.h"
 
+static enum protocol_version *allowed_versions;
+static int nr_allowed_versions;
+static int alloc_allowed_versions;
+static int version_registration_locked = 0;
+
+static const char protocol_v0_string[] = "0";
+static const char protocol_v1_string[] = "1";
+static const char protocol_v2_string[] = "2";
+
 static enum protocol_version parse_protocol_version(const char *value)
 {
-	if (!strcmp(value, "0"))
+	if (!strcmp(value, protocol_v0_string))
 		return protocol_v0;
-	else if (!strcmp(value, "1"))
+	else if (!strcmp(value, protocol_v1_string))
 		return protocol_v1;
-	else if (!strcmp(value, "2"))
+	else if (!strcmp(value, protocol_v2_string))
 		return protocol_v2;
 	else
 		return protocol_unknown_version;
 }
 
+/* Return the text representation of a wire protocol version. */
+static const char *format_protocol_version(enum protocol_version version)
+{
+	switch (version) {
+	case protocol_v0:
+		return protocol_v0_string;
+	case protocol_v1:
+		return protocol_v1_string;
+	case protocol_v2:
+		return protocol_v2_string;
+	case protocol_unknown_version:
+		die(_("Unrecognized protocol version"));
+	}
+	die(_("Unrecognized protocol_version"));
+}
+
 enum protocol_version get_protocol_version_config(void)
 {
 	const char *value;
@@ -30,6 +55,85 @@ enum protocol_version get_protocol_version_config(void)
 	return protocol_v0;
 }
 
+void register_allowed_protocol_version(enum protocol_version version)
+{
+	if (version_registration_locked)
+		BUG("late attempt to register an allowed protocol version");
+
+	ALLOC_GROW(allowed_versions, nr_allowed_versions + 1,
+		   alloc_allowed_versions);
+	allowed_versions[nr_allowed_versions++] = version;
+}
+
+void register_allowed_protocol_versions_from_env(void)
+{
+	const char *git_protocol = getenv(GIT_PROTOCOL_ENVIRONMENT);
+	const char *version_str;
+	struct string_list version_list = STRING_LIST_INIT_DUP;
+	struct string_list_item *version;
+
+	if (!git_protocol)
+		return;
+
+	string_list_split(&version_list, git_protocol, ':', -1);
+	for_each_string_list_item(version, &version_list) {
+		if (skip_prefix(version->string, "version=", &version_str))
+			register_allowed_protocol_version(
+				parse_protocol_version(version_str));
+	}
+	string_list_clear(&version_list, 0);
+}
+
+static int is_allowed_protocol_version(enum protocol_version version)
+{
+	int i;
+	version_registration_locked = 1;
+	for (i = 0; i < nr_allowed_versions; i++)
+		if (version == allowed_versions[i])
+			return 1;
+	return 0;
+}
+
+void get_client_protocol_version_advertisement(struct strbuf *advert)
+{
+	int i, tmp_nr = nr_allowed_versions;
+	enum protocol_version *tmp_allowed_versions, config_version;
+	strbuf_reset(advert);
+
+	version_registration_locked = 1;
+
+	config_version = get_protocol_version_config();
+	if (config_version == protocol_v0) {
+		strbuf_addstr(advert, "version=0");
+		return;
+	}
+
+	if (tmp_nr > 0) {
+		ALLOC_ARRAY(tmp_allowed_versions, tmp_nr);
+		copy_array(tmp_allowed_versions, allowed_versions, tmp_nr,
+			   sizeof(enum protocol_version));
+	} else {
+		ALLOC_ARRAY(tmp_allowed_versions, 1);
+		tmp_nr = 1;
+		tmp_allowed_versions[0] = config_version;
+	}
+
+	if (tmp_allowed_versions[0] != config_version)
+		for (i = 1; i < nr_allowed_versions; i++)
+			if (tmp_allowed_versions[i] == config_version) {
+				SWAP(tmp_allowed_versions[0],
+				     tmp_allowed_versions[i]);
+			}
+
+	strbuf_addf(advert, "version=%s",
+		    format_protocol_version(tmp_allowed_versions[0]));
+	for (i = 1; i < tmp_nr; i++)
+		strbuf_addf(advert, ":version=%s",
+			    format_protocol_version(tmp_allowed_versions[i]));
+
+	free(tmp_allowed_versions);
+}
+
 enum protocol_version determine_protocol_version_server(void)
 {
 	const char *git_protocol = getenv(GIT_PROTOCOL_ENVIRONMENT);
@@ -38,9 +142,10 @@ enum protocol_version determine_protocol_version_server(void)
 	/*
 	 * Determine which protocol version the client has requested.  Since
 	 * multiple 'version' keys can be sent by the client, indicating that
-	 * the client is okay to speak any of them, select the greatest version
-	 * that the client has requested.  This is due to the assumption that
-	 * the most recent protocol version will be the most state-of-the-art.
+	 * the client is okay to speak any of them, select the first
+	 * recognizable version that the client has requested.  This is due to
+	 * the assumption that the protocol versions will be listed in
+	 * preference order.
 	 */
 	if (git_protocol) {
 		struct string_list list = STRING_LIST_INIT_DUP;
@@ -53,8 +158,11 @@ enum protocol_version determine_protocol_version_server(void)
 
 			if (skip_prefix(item->string, "version=", &value)) {
 				v = parse_protocol_version(value);
-				if (v > version)
+				if (v != protocol_unknown_version &&
+				    is_allowed_protocol_version(v)) {
 					version = v;
+					break;
+				}
 			}
 		}
 
diff --git a/protocol.h b/protocol.h
index 2ad35e433c..88330fd0ee 100644
--- a/protocol.h
+++ b/protocol.h
@@ -14,7 +14,24 @@ enum protocol_version {
  * 'protocol.version' config.  If unconfigured, a value of 'protocol_v0' is
  * returned.
  */
-extern enum protocol_version get_protocol_version_config(void);
+enum protocol_version get_protocol_version_config(void);
+
+/*
+ * Register an allowable protocol version for a given operation. Registration
+ * must occur before attempting to advertise a version to a server process.
+ */
+void register_allowed_protocol_version(enum protocol_version version);
+
+/*
+ * Register allowable protocol versions from the GIT_PROTOCOL environment var.
+ */
+void register_allowed_protocol_versions_from_env(void);
+
+/*
+ * Fill a strbuf with a version advertisement string suitable for use in the
+ * GIT_PROTOCOL environment variable or similar version negotiation field.
+ */
+void get_client_protocol_version_advertisement(struct strbuf *advert);
 
 /*
  * Used by a server to determine which protocol version should be used based on
@@ -23,12 +40,12 @@ extern enum protocol_version get_protocol_version_config(void);
  * request a particular protocol version, a default of 'protocol_v0' will be
  * used.
  */
-extern enum protocol_version determine_protocol_version_server(void);
+enum protocol_version determine_protocol_version_server(void);
 
 /*
  * Used by a client to determine which protocol version the server is speaking
  * based on the server's initial response.
  */
-extern enum protocol_version determine_protocol_version_client(const char *server_response);
+enum protocol_version determine_protocol_version_client(const char *server_response);
 
 #endif /* PROTOCOL_H */
diff --git a/remote-curl.c b/remote-curl.c
index fb28309e85..537f8c7aac 100644
--- a/remote-curl.c
+++ b/remote-curl.c
@@ -330,6 +330,20 @@ static int get_protocol_http_header(enum protocol_version version,
 	return 0;
 }
 
+static int get_client_protocol_http_header(const struct strbuf *version_advert,
+					   struct strbuf *header)
+{
+	if (version_advert->len > 0 &&
+	    strcmp(version_advert->buf, "version=0")) {
+		strbuf_addf(header, GIT_PROTOCOL_HEADER ": %s",
+			    version_advert->buf);
+
+		return 1;
+	}
+
+	return 0;
+}
+
 static struct discovery *discover_refs(const char *service, int for_push)
 {
 	struct strbuf exp = STRBUF_INIT;
@@ -339,11 +353,11 @@ static struct discovery *discover_refs(const char *service, int for_push)
 	struct strbuf refs_url = STRBUF_INIT;
 	struct strbuf effective_url = STRBUF_INIT;
 	struct strbuf protocol_header = STRBUF_INIT;
+	struct strbuf version_advert = STRBUF_INIT;
 	struct string_list extra_headers = STRING_LIST_INIT_DUP;
 	struct discovery *last = last_discovery;
 	int http_ret, maybe_smart = 0;
 	struct http_get_options http_options;
-	enum protocol_version version = get_protocol_version_config();
 
 	if (last && !strcmp(service, last->service))
 		return last;
@@ -360,16 +374,10 @@ static struct discovery *discover_refs(const char *service, int for_push)
 		strbuf_addf(&refs_url, "service=%s", service);
 	}
 
-	/*
-	 * NEEDSWORK: If we are trying to use protocol v2 and we are planning
-	 * to perform a push, then fallback to v0 since the client doesn't know
-	 * how to push yet using v2.
-	 */
-	if (version == protocol_v2 && !strcmp("git-receive-pack", service))
-		version = protocol_v0;
+	get_client_protocol_version_advertisement(&version_advert);
 
 	/* Add the extra Git-Protocol header */
-	if (get_protocol_http_header(version, &protocol_header))
+	if (get_client_protocol_http_header(&version_advert, &protocol_header))
 		string_list_append(&extra_headers, protocol_header.buf);
 
 	memset(&http_options, 0, sizeof(http_options));
@@ -1327,6 +1335,8 @@ int cmd_main(int argc, const char **argv)
 	struct strbuf buf = STRBUF_INIT;
 	int nongit;
 
+	register_allowed_protocol_versions_from_env();
+
 	setup_git_directory_gently(&nongit);
 	if (argc < 2) {
 		error("remote-curl: usage: git remote-curl <remote> [<url>]");
diff --git a/t/t5570-git-daemon.sh b/t/t5570-git-daemon.sh
index 7466aad111..d528e91630 100755
--- a/t/t5570-git-daemon.sh
+++ b/t/t5570-git-daemon.sh
@@ -186,7 +186,7 @@ test_expect_success 'hostname cannot break out of directory' '
 test_expect_success 'daemon log records all attributes' '
 	cat >expect <<-\EOF &&
 	Extended attribute "host": localhost
-	Extended attribute "protocol": version=1
+	Extended attribute "protocol": version=1:version=2:version=0
 	EOF
 	>daemon.log &&
 	GIT_OVERRIDE_VIRTUAL_HOST=localhost \
diff --git a/t/t5700-protocol-v1.sh b/t/t5700-protocol-v1.sh
index ba86a44eb1..2e56c79233 100755
--- a/t/t5700-protocol-v1.sh
+++ b/t/t5700-protocol-v1.sh
@@ -26,7 +26,7 @@ test_expect_success 'clone with git:// using protocol v1' '
 	test_cmp expect actual &&
 
 	# Client requested to use protocol v1
-	grep "clone> .*\\\0\\\0version=1\\\0$" log &&
+	grep "clone> .*\\\0\\\0version=1.*\\\0$" log &&
 	# Server responded using protocol v1
 	grep "clone< version 1" log
 '
@@ -42,7 +42,7 @@ test_expect_success 'fetch with git:// using protocol v1' '
 	test_cmp expect actual &&
 
 	# Client requested to use protocol v1
-	grep "fetch> .*\\\0\\\0version=1\\\0$" log &&
+	grep "fetch> .*\\\0\\\0version=1.*\\\0$" log &&
 	# Server responded using protocol v1
 	grep "fetch< version 1" log
 '
@@ -56,7 +56,7 @@ test_expect_success 'pull with git:// using protocol v1' '
 	test_cmp expect actual &&
 
 	# Client requested to use protocol v1
-	grep "fetch> .*\\\0\\\0version=1\\\0$" log &&
+	grep "fetch> .*\\\0\\\0version=1.*\\\0$" log &&
 	# Server responded using protocol v1
 	grep "fetch< version 1" log
 '
@@ -74,7 +74,7 @@ test_expect_success 'push with git:// using protocol v1' '
 	test_cmp expect actual &&
 
 	# Client requested to use protocol v1
-	grep "push> .*\\\0\\\0version=1\\\0$" log &&
+	grep "push> .*\\\0\\\0version=1.*\\\0$" log &&
 	# Server responded using protocol v1
 	grep "push< version 1" log
 '
diff --git a/t/t5702-protocol-v2.sh b/t/t5702-protocol-v2.sh
index 3beeed4546..78c17c25e4 100755
--- a/t/t5702-protocol-v2.sh
+++ b/t/t5702-protocol-v2.sh
@@ -24,7 +24,7 @@ test_expect_success 'list refs with git:// using protocol v2' '
 		ls-remote --symref "$GIT_DAEMON_URL/parent" >actual &&
 
 	# Client requested to use protocol v2
-	grep "git> .*\\\0\\\0version=2\\\0$" log &&
+	grep "git> .*\\\0\\\0version=2.*\\\0$" log &&
 	# Server responded using protocol v2
 	grep "git< version 2" log &&
 
@@ -56,7 +56,7 @@ test_expect_success 'clone with git:// using protocol v2' '
 	test_cmp expect actual &&
 
 	# Client requested to use protocol v2
-	grep "clone> .*\\\0\\\0version=2\\\0$" log &&
+	grep "clone> .*\\\0\\\0version=2.*\\\0$" log &&
 	# Server responded using protocol v2
 	grep "clone< version 2" log
 '
@@ -74,7 +74,7 @@ test_expect_success 'fetch with git:// using protocol v2' '
 	test_cmp expect actual &&
 
 	# Client requested to use protocol v2
-	grep "fetch> .*\\\0\\\0version=2\\\0$" log &&
+	grep "fetch> .*\\\0\\\0version=2.*\\\0$" log &&
 	# Server responded using protocol v2
 	grep "fetch< version 2" log
 '
@@ -90,7 +90,7 @@ test_expect_success 'pull with git:// using protocol v2' '
 	test_cmp expect actual &&
 
 	# Client requested to use protocol v2
-	grep "fetch> .*\\\0\\\0version=2\\\0$" log &&
+	grep "fetch> .*\\\0\\\0version=2.*\\\0$" log &&
 	# Server responded using protocol v2
 	grep "fetch< version 2" log
 '
@@ -476,7 +476,7 @@ test_expect_success 'push with http:// and a config of v2 does not request v2' '
 	test_when_finished "rm -f log" &&
 	# Till v2 for push is designed, make sure that if a client has
 	# protocol.version configured to use v2, that the client instead falls
-	# back and uses v0.
+	# back to previous versions.
 
 	test_commit -C http_child three &&
 
@@ -489,10 +489,8 @@ test_expect_success 'push with http:// and a config of v2 does not request v2' '
 	git -C "$HTTPD_DOCUMENT_ROOT_PATH/http_parent" log -1 --format=%s client_branch >expect &&
 	test_cmp expect actual &&
 
-	# Client didnt request to use protocol v2
-	! grep "Git-Protocol: version=2" log &&
-	# Server didnt respond using protocol v2
-	! grep "git< version 2" log
+	# Server responded with version 1
+	grep "git< version 1" log
 '
 
 
diff --git a/transport-helper.c b/transport-helper.c
index 143ca008c8..ac1937f1e1 100644
--- a/transport-helper.c
+++ b/transport-helper.c
@@ -105,6 +105,7 @@ static struct child_process *get_helper(struct transport *transport)
 {
 	struct helper_data *data = transport->data;
 	struct strbuf buf = STRBUF_INIT;
+	struct strbuf version_advert = STRBUF_INIT;
 	struct child_process *helper;
 	int duped;
 	int code;
@@ -127,6 +128,11 @@ static struct child_process *get_helper(struct transport *transport)
 		argv_array_pushf(&helper->env_array, "%s=%s",
 				 GIT_DIR_ENVIRONMENT, get_git_dir());
 
+	get_client_protocol_version_advertisement(&version_advert);
+	if (version_advert.len > 0)
+		argv_array_pushf(&helper->env_array, "%s=%s",
+				 GIT_PROTOCOL_ENVIRONMENT, version_advert.buf);
+
 	code = start_command(helper);
 	if (code < 0 && errno == ENOENT)
 		die(_("unable to find remote helper for '%s'"), data->name);
-- 
2.19.1.930.g4563a0d9d0-goog

Re: [PATCH v4 0/1] Advertise multiple supported proto versions

[Junio C Hamano]

Josh Steadmon <steadmon@google.com> writes:

> Fix several bugs identified in v3, clarify commit message, and clean up
> extern keyword in protocol.h.

It is good to descirbe the change relative to v3 here, which would
help those who are interested and reviewed v3.

To help those who missed the boat and v4 is their first encounter
with this series, having the description relative to v3 alone and
nothing else is not very friendly, though.

>     + diff --git a/builtin/upload-archive.c b/builtin/upload-archive.c
>     + --- a/builtin/upload-archive.c
>     + +++ b/builtin/upload-archive.c
>     +@@
>     + #include "builtin.h"
>     + #include "archive.h"
>     + #include "pkt-line.h"
>     ++#include "protocol.h"
>     + #include "sideband.h"
>     + #include "run-command.h"
>     + #include "argv-array.h"
>     +@@
>     + 	if (argc == 2 && !strcmp(argv[1], "-h"))
>     + 		usage(upload_archive_usage);
>     + 
>     ++	register_allowed_protocol_version(protocol_v0);
>     ++
>     + 	/*
>     + 	 * Set up sideband subprocess.
>     + 	 *

This one unfortunately seems to interact rather badly with your
js/remote-archive-v2 topic which is already in 'next', when this
topic is merged to 'pu', and my attempt to mechanically resolve
conflicts breaks t5000.

Re: [PATCH v4 0/1] Advertise multiple supported proto versions

[Josh Steadmon]

On 2018.11.14 19:22, Junio C Hamano wrote:
> Josh Steadmon <steadmon@google.com> writes:
> 
> > Fix several bugs identified in v3, clarify commit message, and clean up
> > extern keyword in protocol.h.
> 
> It is good to descirbe the change relative to v3 here, which would
> help those who are interested and reviewed v3.
> 
> To help those who missed the boat and v4 is their first encounter
> with this series, having the description relative to v3 alone and
> nothing else is not very friendly, though.

Ack. Will keep this in mind for future patches.

> >     + diff --git a/builtin/upload-archive.c b/builtin/upload-archive.c
> >     + --- a/builtin/upload-archive.c
> >     + +++ b/builtin/upload-archive.c
> >     +@@
> >     + #include "builtin.h"
> >     + #include "archive.h"
> >     + #include "pkt-line.h"
> >     ++#include "protocol.h"
> >     + #include "sideband.h"
> >     + #include "run-command.h"
> >     + #include "argv-array.h"
> >     +@@
> >     + 	if (argc == 2 && !strcmp(argv[1], "-h"))
> >     + 		usage(upload_archive_usage);
> >     + 
> >     ++	register_allowed_protocol_version(protocol_v0);
> >     ++
> >     + 	/*
> >     + 	 * Set up sideband subprocess.
> >     + 	 *
> 
> This one unfortunately seems to interact rather badly with your
> js/remote-archive-v2 topic which is already in 'next', when this
> topic is merged to 'pu', and my attempt to mechanically resolve
> conflicts breaks t5000.

Hmm, should we drop js/remote-archive-v2 then? Any solution that
unblocks js/remote-archive-v2 will almost certainly depend on this
patch.

Re: [PATCH v4 0/1] Advertise multiple supported proto versions

[Junio C Hamano]

Josh Steadmon <steadmon@google.com> writes:

>> This one unfortunately seems to interact rather badly with your
>> js/remote-archive-v2 topic which is already in 'next', when this
>> topic is merged to 'pu', and my attempt to mechanically resolve
>> conflicts breaks t5000.
>
> Hmm, should we drop js/remote-archive-v2 then? Any solution that
> unblocks js/remote-archive-v2 will almost certainly depend on this
> patch.

That one is already in 'next' for quite some time.  If you are
retrating it, that's OK.

Let me revert the topic out of 'next' and discard it, and then queue
this one in 'pu'.

Thanks.

[PATCH v5 0/1] Advertise multiple supported proto versions

[Josh Steadmon]

Changes from V4: remove special cases around advertising version=0.

Add a registry of supported wire protocol versions that individual
commands can use to declare supported versions before contacting a
server. The client will then advertise all supported versions, while the
server will choose the first allowed version from the advertised
list.

Every command that acts as a client or server must now register its
supported protocol versions.


Josh Steadmon (1):
  protocol: advertise multiple supported versions

 builtin/archive.c           |   3 +
 builtin/clone.c             |   4 ++
 builtin/fetch-pack.c        |   4 ++
 builtin/fetch.c             |   5 ++
 builtin/ls-remote.c         |   5 ++
 builtin/pull.c              |   5 ++
 builtin/push.c              |   4 ++
 builtin/receive-pack.c      |   3 +
 builtin/send-pack.c         |   3 +
 builtin/upload-archive.c    |   3 +
 builtin/upload-pack.c       |   4 ++
 connect.c                   |  52 +++++++--------
 protocol.c                  | 122 +++++++++++++++++++++++++++++++++---
 protocol.h                  |  23 ++++++-
 remote-curl.c               |  27 +++++---
 t/t5551-http-fetch-smart.sh |   1 +
 t/t5570-git-daemon.sh       |   2 +-
 t/t5601-clone.sh            |  38 +++++------
 t/t5700-protocol-v1.sh      |   8 +--
 t/t5702-protocol-v2.sh      |  16 +++--
 transport-helper.c          |   6 ++
 21 files changed, 256 insertions(+), 82 deletions(-)

Range-diff against v4:
1:  3c023991c0 ! 1:  60f6f2fbd8 protocol: advertise multiple supported versions
    @@ -21,6 +21,12 @@
         Clients now advertise the full list of registered versions. Servers
         select the first allowed version from this advertisement.
     
    +    Additionally, remove special cases around advertising version=0.
    +    Previously we avoided adding version negotiation fields in server
    +    responses if it looked like the client wanted v0. However, including
    +    these fields does not change behavior, so it's better to have simpler
    +    code.
    +
         While we're at it, remove unnecessary externs from function declarations
         in protocol.h.
     
    @@ -245,18 +251,21 @@
      {
      	struct child_process *conn;
     @@
    + 		    prog, path, 0,
      		    target_host, 0);
      
    - 	/* If using a new version put that stuff here after a second null byte */
    +-	/* If using a new version put that stuff here after a second null byte */
     -	if (version > 0) {
    -+	if (strcmp(version_advert->buf, "version=0")) {
    - 		strbuf_addch(&request, '\0');
    +-		strbuf_addch(&request, '\0');
     -		strbuf_addf(&request, "version=%d%c",
     -			    version, '\0');
    -+		strbuf_addf(&request, "%s%c", version_advert->buf, '\0');
    - 	}
    +-	}
    ++	/* Add version fields after a second null byte */
    ++	strbuf_addch(&request, '\0');
    ++	strbuf_addf(&request, "%s%c", version_advert->buf, '\0');
      
      	packet_write(fd[1], request.buf, request.len);
    + 
     @@
       */
      static void push_ssh_options(struct argv_array *args, struct argv_array *env,
    @@ -264,9 +273,9 @@
     -			     enum protocol_version version, int flags)
     +			     const struct strbuf *version_advert, int flags)
      {
    - 	if (variant == VARIANT_SSH &&
    +-	if (variant == VARIANT_SSH &&
     -	    version > 0) {
    -+	    strcmp(version_advert->buf, "version=0")) {
    ++	if (variant == VARIANT_SSH) {
      		argv_array_push(args, "-o");
      		argv_array_push(args, "SendEnv=" GIT_PROTOCOL_ENVIRONMENT);
     -		argv_array_pushf(env, GIT_PROTOCOL_ENVIRONMENT "=version=%d",
    @@ -346,13 +355,13 @@
     -			if (version > 0) {
     -				argv_array_pushf(&conn->env_array, GIT_PROTOCOL_ENVIRONMENT "=version=%d",
     -						 version);
    -+			if (strcmp(version_advert.buf, "version=0")) {
    -+				argv_array_pushf(&conn->env_array,
    -+						 GIT_PROTOCOL_ENVIRONMENT "=%s",
    -+						 version_advert.buf);
    - 			}
    +-			}
    ++			argv_array_pushf(&conn->env_array,
    ++					 GIT_PROTOCOL_ENVIRONMENT "=%s",
    ++					 version_advert.buf);
      		}
      		argv_array_push(&conn->args, cmd.buf);
    + 
     
      diff --git a/protocol.c b/protocol.c
      --- a/protocol.c
    @@ -573,8 +582,7 @@
     +static int get_client_protocol_http_header(const struct strbuf *version_advert,
     +					   struct strbuf *header)
     +{
    -+	if (version_advert->len > 0 &&
    -+	    strcmp(version_advert->buf, "version=0")) {
    ++	if (version_advert->len > 0) {
     +		strbuf_addf(header, GIT_PROTOCOL_HEADER ": %s",
     +			    version_advert->buf);
     +
    @@ -629,6 +637,18 @@
      	if (argc < 2) {
      		error("remote-curl: usage: git remote-curl <remote> [<url>]");
     
    + diff --git a/t/t5551-http-fetch-smart.sh b/t/t5551-http-fetch-smart.sh
    + --- a/t/t5551-http-fetch-smart.sh
    + +++ b/t/t5551-http-fetch-smart.sh
    +@@
    + > Accept: */*
    + > Accept-Encoding: ENCODINGS
    + > Pragma: no-cache
    ++> Git-Protocol: version=0
    + < HTTP/1.1 200 OK
    + < Pragma: no-cache
    + < Cache-Control: no-cache, max-age=0, must-revalidate
    +
      diff --git a/t/t5570-git-daemon.sh b/t/t5570-git-daemon.sh
      --- a/t/t5570-git-daemon.sh
      +++ b/t/t5570-git-daemon.sh
    @@ -642,6 +662,161 @@
      	>daemon.log &&
      	GIT_OVERRIDE_VIRTUAL_HOST=localhost \
     
    + diff --git a/t/t5601-clone.sh b/t/t5601-clone.sh
    + --- a/t/t5601-clone.sh
    + +++ b/t/t5601-clone.sh
    +@@
    + 
    + test_expect_success 'clone myhost:src uses ssh' '
    + 	git clone myhost:src ssh-clone &&
    +-	expect_ssh myhost src
    ++	expect_ssh "-o SendEnv=GIT_PROTOCOL" myhost src
    + '
    + 
    + test_expect_success !MINGW,!CYGWIN 'clone local path foo:bar' '
    +@@
    + 
    + test_expect_success 'bracketed hostnames are still ssh' '
    + 	git clone "[myhost:123]:src" ssh-bracket-clone &&
    +-	expect_ssh "-p 123" myhost src
    ++	expect_ssh "-o SendEnv=GIT_PROTOCOL -p 123" myhost src
    + '
    + 
    + test_expect_success 'OpenSSH variant passes -4' '
    + 	git clone -4 "[myhost:123]:src" ssh-ipv4-clone &&
    +-	expect_ssh "-4 -p 123" myhost src
    ++	expect_ssh "-o SendEnv=GIT_PROTOCOL -4 -p 123" myhost src
    + '
    + 
    + test_expect_success 'variant can be overridden' '
    +@@
    + 	GIT_SSH="$TRASH_DIRECTORY/uplink" &&
    + 	test_when_finished "GIT_SSH=\"\$TRASH_DIRECTORY/ssh\$X\"" &&
    + 	git clone "[myhost:123]:src" ssh-bracket-clone-sshlike-uplink &&
    +-	expect_ssh "-p 123" myhost src
    ++	expect_ssh "-o SendEnv=GIT_PROTOCOL -p 123" myhost src
    + '
    + 
    + test_expect_success 'plink is treated specially (as putty)' '
    +@@
    + 	copy_ssh_wrapper_as "$TRASH_DIRECTORY/plink" &&
    + 	GIT_SSH_VARIANT=ssh \
    + 	git clone "[myhost:123]:src" ssh-bracket-clone-variant-1 &&
    +-	expect_ssh "-p 123" myhost src
    ++	expect_ssh "-o SendEnv=GIT_PROTOCOL -p 123" myhost src
    + '
    + 
    + test_expect_success 'ssh.variant overrides plink detection' '
    + 	copy_ssh_wrapper_as "$TRASH_DIRECTORY/plink" &&
    + 	git -c ssh.variant=ssh \
    + 		clone "[myhost:123]:src" ssh-bracket-clone-variant-2 &&
    +-	expect_ssh "-p 123" myhost src
    ++	expect_ssh "-o SendEnv=GIT_PROTOCOL -p 123" myhost src
    + '
    + 
    + test_expect_success 'GIT_SSH_VARIANT overrides plink detection to plink' '
    +@@
    + }
    + 
    + test_expect_success !MINGW 'clone c:temp is ssl' '
    +-	test_clone_url c:temp c temp
    ++	test_clone_url c:temp "-o SendEnv=GIT_PROTOCOL" c temp
    + '
    + 
    + test_expect_success MINGW 'clone c:temp is dos drive' '
    +@@
    + for repo in rep rep/home/project 123
    + do
    + 	test_expect_success "clone host:$repo" '
    +-		test_clone_url host:$repo host $repo
    ++		test_clone_url host:$repo "-o SendEnv=GIT_PROTOCOL" host $repo
    + 	'
    + done
    + 
    +@@
    + for repo in rep rep/home/project 123
    + do
    + 	test_expect_success "clone [::1]:$repo" '
    +-		test_clone_url [::1]:$repo ::1 "$repo"
    ++		test_clone_url [::1]:$repo "-o SendEnv=GIT_PROTOCOL" ::1 "$repo"
    + 	'
    + done
    + #home directory
    + test_expect_success "clone host:/~repo" '
    +-	test_clone_url host:/~repo host "~repo"
    ++	test_clone_url host:/~repo "-o SendEnv=GIT_PROTOCOL" host "~repo"
    + '
    + 
    + test_expect_success "clone [::1]:/~repo" '
    +-	test_clone_url [::1]:/~repo ::1 "~repo"
    ++	test_clone_url [::1]:/~repo "-o SendEnv=GIT_PROTOCOL" ::1 "~repo"
    + '
    + 
    + # Corner cases
    +@@
    + for tcol in "" :
    + do
    + 	test_expect_success "clone ssh://host.xz$tcol/home/user/repo" '
    +-		test_clone_url "ssh://host.xz$tcol/home/user/repo" host.xz /home/user/repo
    ++		test_clone_url "ssh://host.xz$tcol/home/user/repo" "-o SendEnv=GIT_PROTOCOL" host.xz /home/user/repo
    + 	'
    + 	# from home directory
    + 	test_expect_success "clone ssh://host.xz$tcol/~repo" '
    +-	test_clone_url "ssh://host.xz$tcol/~repo" host.xz "~repo"
    ++	test_clone_url "ssh://host.xz$tcol/~repo" "-o SendEnv=GIT_PROTOCOL" host.xz "~repo"
    + '
    + done
    + 
    + # with port number
    + test_expect_success 'clone ssh://host.xz:22/home/user/repo' '
    +-	test_clone_url "ssh://host.xz:22/home/user/repo" "-p 22 host.xz" "/home/user/repo"
    ++	test_clone_url "ssh://host.xz:22/home/user/repo" "-o SendEnv=GIT_PROTOCOL -p 22 host.xz" "/home/user/repo"
    + '
    + 
    + # from home directory with port number
    + test_expect_success 'clone ssh://host.xz:22/~repo' '
    +-	test_clone_url "ssh://host.xz:22/~repo" "-p 22 host.xz" "~repo"
    ++	test_clone_url "ssh://host.xz:22/~repo" "-o SendEnv=GIT_PROTOCOL -p 22 host.xz" "~repo"
    + '
    + 
    + #IPv6
    +@@
    + do
    + 	ehost=$(echo $tuah | sed -e "s/1]:/1]/" | tr -d "[]")
    + 	test_expect_success "clone ssh://$tuah/home/user/repo" "
    +-	  test_clone_url ssh://$tuah/home/user/repo $ehost /home/user/repo
    ++	  test_clone_url ssh://$tuah/home/user/repo '-o SendEnv=GIT_PROTOCOL' $ehost /home/user/repo
    + 	"
    + done
    + 
    +@@
    + do
    + 	euah=$(echo $tuah | tr -d "[]")
    + 	test_expect_success "clone ssh://$tuah/~repo" "
    +-	  test_clone_url ssh://$tuah/~repo $euah '~repo'
    ++	  test_clone_url ssh://$tuah/~repo '-o SendEnv=GIT_PROTOCOL' $euah '~repo'
    + 	"
    + done
    + 
    +@@
    + do
    + 	euah=$(echo $tuah | tr -d "[]")
    + 	test_expect_success "clone ssh://$tuah:22/home/user/repo" "
    +-	  test_clone_url ssh://$tuah:22/home/user/repo '-p 22' $euah /home/user/repo
    ++	  test_clone_url ssh://$tuah:22/home/user/repo '-o SendEnv=GIT_PROTOCOL -p 22' $euah /home/user/repo
    + 	"
    + done
    + 
    +@@
    + do
    + 	euah=$(echo $tuah | tr -d "[]")
    + 	test_expect_success "clone ssh://$tuah:22/~repo" "
    +-	  test_clone_url ssh://$tuah:22/~repo '-p 22' $euah '~repo'
    ++	  test_clone_url ssh://$tuah:22/~repo '-o SendEnv=GIT_PROTOCOL -p 22' $euah '~repo'
    + 	"
    + done
    + 
    +
      diff --git a/t/t5700-protocol-v1.sh b/t/t5700-protocol-v1.sh
      --- a/t/t5700-protocol-v1.sh
      +++ b/t/t5700-protocol-v1.sh
-- 
2.19.1.1215.g8438c0b245-goog

[PATCH v5 1/1] protocol: advertise multiple supported versions

[Josh Steadmon]

Currently the client advertises that it supports the wire protocol
version set in the protocol.version config. However, not all services
support the same set of protocol versions. For example, git-receive-pack
supports v1 and v0, but not v2. If a client connects to git-receive-pack
and requests v2, it will instead be downgraded to v0. Other services,
such as git-upload-archive, do not do any version negotiation checks.

This patch creates a protocol version registry. Individual client and
server programs register all the protocol versions they support prior to
communicating with a remote instance. Versions should be listed in
preference order; the version specified in protocol.version will
automatically be moved to the front of the registry.

The protocol version registry is passed to remote helpers via the
GIT_PROTOCOL environment variable.

Clients now advertise the full list of registered versions. Servers
select the first allowed version from this advertisement.

Additionally, remove special cases around advertising version=0.
Previously we avoided adding version negotiation fields in server
responses if it looked like the client wanted v0. However, including
these fields does not change behavior, so it's better to have simpler
code.

While we're at it, remove unnecessary externs from function declarations
in protocol.h.

Signed-off-by: Josh Steadmon <steadmon@google.com>
---
 builtin/archive.c           |   3 +
 builtin/clone.c             |   4 ++
 builtin/fetch-pack.c        |   4 ++
 builtin/fetch.c             |   5 ++
 builtin/ls-remote.c         |   5 ++
 builtin/pull.c              |   5 ++
 builtin/push.c              |   4 ++
 builtin/receive-pack.c      |   3 +
 builtin/send-pack.c         |   3 +
 builtin/upload-archive.c    |   3 +
 builtin/upload-pack.c       |   4 ++
 connect.c                   |  52 +++++++--------
 protocol.c                  | 122 +++++++++++++++++++++++++++++++++---
 protocol.h                  |  23 ++++++-
 remote-curl.c               |  27 +++++---
 t/t5551-http-fetch-smart.sh |   1 +
 t/t5570-git-daemon.sh       |   2 +-
 t/t5601-clone.sh            |  38 +++++------
 t/t5700-protocol-v1.sh      |   8 +--
 t/t5702-protocol-v2.sh      |  16 +++--
 transport-helper.c          |   6 ++
 21 files changed, 256 insertions(+), 82 deletions(-)

diff --git a/builtin/archive.c b/builtin/archive.c
index e74f675390..8adb9f381b 100644
--- a/builtin/archive.c
+++ b/builtin/archive.c
@@ -8,6 +8,7 @@
 #include "transport.h"
 #include "parse-options.h"
 #include "pkt-line.h"
+#include "protocol.h"
 #include "sideband.h"
 
 static void create_output_file(const char *output_file)
@@ -94,6 +95,8 @@ int cmd_archive(int argc, const char **argv, const char *prefix)
 		OPT_END()
 	};
 
+	register_allowed_protocol_version(protocol_v0);
+
 	argc = parse_options(argc, argv, prefix, local_opts, NULL,
 			     PARSE_OPT_KEEP_ALL);
 
diff --git a/builtin/clone.c b/builtin/clone.c
index fd2c3ef090..1651a950b6 100644
--- a/builtin/clone.c
+++ b/builtin/clone.c
@@ -900,6 +900,10 @@ int cmd_clone(int argc, const char **argv, const char *prefix)
 	struct refspec rs = REFSPEC_INIT_FETCH;
 	struct argv_array ref_prefixes = ARGV_ARRAY_INIT;
 
+	register_allowed_protocol_version(protocol_v2);
+	register_allowed_protocol_version(protocol_v1);
+	register_allowed_protocol_version(protocol_v0);
+
 	fetch_if_missing = 0;
 
 	packet_trace_identity("clone");
diff --git a/builtin/fetch-pack.c b/builtin/fetch-pack.c
index 1a1bc63566..cba935e4d3 100644
--- a/builtin/fetch-pack.c
+++ b/builtin/fetch-pack.c
@@ -57,6 +57,10 @@ int cmd_fetch_pack(int argc, const char **argv, const char *prefix)
 
 	fetch_if_missing = 0;
 
+	register_allowed_protocol_version(protocol_v2);
+	register_allowed_protocol_version(protocol_v1);
+	register_allowed_protocol_version(protocol_v0);
+
 	packet_trace_identity("fetch-pack");
 
 	memset(&args, 0, sizeof(args));
diff --git a/builtin/fetch.c b/builtin/fetch.c
index 61bec5d213..2a20cf8bfd 100644
--- a/builtin/fetch.c
+++ b/builtin/fetch.c
@@ -21,6 +21,7 @@
 #include "argv-array.h"
 #include "utf8.h"
 #include "packfile.h"
+#include "protocol.h"
 #include "list-objects-filter-options.h"
 
 static const char * const builtin_fetch_usage[] = {
@@ -1476,6 +1477,10 @@ int cmd_fetch(int argc, const char **argv, const char *prefix)
 	int prune_tags_ok = 1;
 	struct argv_array argv_gc_auto = ARGV_ARRAY_INIT;
 
+	register_allowed_protocol_version(protocol_v2);
+	register_allowed_protocol_version(protocol_v1);
+	register_allowed_protocol_version(protocol_v0);
+
 	packet_trace_identity("fetch");
 
 	fetch_if_missing = 0;
diff --git a/builtin/ls-remote.c b/builtin/ls-remote.c
index 1a25df7ee1..ea685e8bb9 100644
--- a/builtin/ls-remote.c
+++ b/builtin/ls-remote.c
@@ -1,5 +1,6 @@
 #include "builtin.h"
 #include "cache.h"
+#include "protocol.h"
 #include "transport.h"
 #include "ref-filter.h"
 #include "remote.h"
@@ -80,6 +81,10 @@ int cmd_ls_remote(int argc, const char **argv, const char *prefix)
 
 	memset(&ref_array, 0, sizeof(ref_array));
 
+	register_allowed_protocol_version(protocol_v2);
+	register_allowed_protocol_version(protocol_v1);
+	register_allowed_protocol_version(protocol_v0);
+
 	argc = parse_options(argc, argv, prefix, options, ls_remote_usage,
 			     PARSE_OPT_STOP_AT_NON_OPTION);
 	dest = argv[0];
diff --git a/builtin/pull.c b/builtin/pull.c
index 681c127a07..cb64146834 100644
--- a/builtin/pull.c
+++ b/builtin/pull.c
@@ -9,6 +9,7 @@
 #include "config.h"
 #include "builtin.h"
 #include "parse-options.h"
+#include "protocol.h"
 #include "exec-cmd.h"
 #include "run-command.h"
 #include "sha1-array.h"
@@ -849,6 +850,10 @@ int cmd_pull(int argc, const char **argv, const char *prefix)
 	struct object_id rebase_fork_point;
 	int autostash;
 
+	register_allowed_protocol_version(protocol_v2);
+	register_allowed_protocol_version(protocol_v1);
+	register_allowed_protocol_version(protocol_v0);
+
 	if (!getenv("GIT_REFLOG_ACTION"))
 		set_reflog_message(argc, argv);
 
diff --git a/builtin/push.c b/builtin/push.c
index d09a42062c..10d8abe829 100644
--- a/builtin/push.c
+++ b/builtin/push.c
@@ -10,6 +10,7 @@
 #include "remote.h"
 #include "transport.h"
 #include "parse-options.h"
+#include "protocol.h"
 #include "submodule.h"
 #include "submodule-config.h"
 #include "send-pack.h"
@@ -587,6 +588,9 @@ int cmd_push(int argc, const char **argv, const char *prefix)
 		OPT_END()
 	};
 
+	register_allowed_protocol_version(protocol_v1);
+	register_allowed_protocol_version(protocol_v0);
+
 	packet_trace_identity("push");
 	git_config(git_push_config, &flags);
 	argc = parse_options(argc, argv, prefix, options, push_usage, 0);
diff --git a/builtin/receive-pack.c b/builtin/receive-pack.c
index c17ce94e12..030cb7b7ec 100644
--- a/builtin/receive-pack.c
+++ b/builtin/receive-pack.c
@@ -1929,6 +1929,9 @@ int cmd_receive_pack(int argc, const char **argv, const char *prefix)
 
 	packet_trace_identity("receive-pack");
 
+	register_allowed_protocol_version(protocol_v1);
+	register_allowed_protocol_version(protocol_v0);
+
 	argc = parse_options(argc, argv, prefix, options, receive_pack_usage, 0);
 
 	if (argc > 1)
diff --git a/builtin/send-pack.c b/builtin/send-pack.c
index 8e3c7490f7..f48bd1306b 100644
--- a/builtin/send-pack.c
+++ b/builtin/send-pack.c
@@ -184,6 +184,9 @@ int cmd_send_pack(int argc, const char **argv, const char *prefix)
 		OPT_END()
 	};
 
+	register_allowed_protocol_version(protocol_v1);
+	register_allowed_protocol_version(protocol_v0);
+
 	git_config(send_pack_config, NULL);
 	argc = parse_options(argc, argv, prefix, options, send_pack_usage, 0);
 	if (argc > 0) {
diff --git a/builtin/upload-archive.c b/builtin/upload-archive.c
index 25d9116356..791cbe80a6 100644
--- a/builtin/upload-archive.c
+++ b/builtin/upload-archive.c
@@ -5,6 +5,7 @@
 #include "builtin.h"
 #include "archive.h"
 #include "pkt-line.h"
+#include "protocol.h"
 #include "sideband.h"
 #include "run-command.h"
 #include "argv-array.h"
@@ -80,6 +81,8 @@ int cmd_upload_archive(int argc, const char **argv, const char *prefix)
 	if (argc == 2 && !strcmp(argv[1], "-h"))
 		usage(upload_archive_usage);
 
+	register_allowed_protocol_version(protocol_v0);
+
 	/*
 	 * Set up sideband subprocess.
 	 *
diff --git a/builtin/upload-pack.c b/builtin/upload-pack.c
index 42dc4da5a1..293dd45b9e 100644
--- a/builtin/upload-pack.c
+++ b/builtin/upload-pack.c
@@ -33,6 +33,10 @@ int cmd_upload_pack(int argc, const char **argv, const char *prefix)
 	packet_trace_identity("upload-pack");
 	read_replace_refs = 0;
 
+	register_allowed_protocol_version(protocol_v2);
+	register_allowed_protocol_version(protocol_v1);
+	register_allowed_protocol_version(protocol_v0);
+
 	argc = parse_options(argc, argv, NULL, options, upload_pack_usage, 0);
 
 	if (argc != 1)
diff --git a/connect.c b/connect.c
index 94547e5056..57266b6cec 100644
--- a/connect.c
+++ b/connect.c
@@ -1046,7 +1046,7 @@ static enum ssh_variant determine_ssh_variant(const char *ssh_command,
  */
 static struct child_process *git_connect_git(int fd[2], char *hostandport,
 					     const char *path, const char *prog,
-					     enum protocol_version version,
+					     const struct strbuf *version_advert,
 					     int flags)
 {
 	struct child_process *conn;
@@ -1084,12 +1084,9 @@ static struct child_process *git_connect_git(int fd[2], char *hostandport,
 		    prog, path, 0,
 		    target_host, 0);
 
-	/* If using a new version put that stuff here after a second null byte */
-	if (version > 0) {
-		strbuf_addch(&request, '\0');
-		strbuf_addf(&request, "version=%d%c",
-			    version, '\0');
-	}
+	/* Add version fields after a second null byte */
+	strbuf_addch(&request, '\0');
+	strbuf_addf(&request, "%s%c", version_advert->buf, '\0');
 
 	packet_write(fd[1], request.buf, request.len);
 
@@ -1104,14 +1101,13 @@ static struct child_process *git_connect_git(int fd[2], char *hostandport,
  */
 static void push_ssh_options(struct argv_array *args, struct argv_array *env,
 			     enum ssh_variant variant, const char *port,
-			     enum protocol_version version, int flags)
+			     const struct strbuf *version_advert, int flags)
 {
-	if (variant == VARIANT_SSH &&
-	    version > 0) {
+	if (variant == VARIANT_SSH) {
 		argv_array_push(args, "-o");
 		argv_array_push(args, "SendEnv=" GIT_PROTOCOL_ENVIRONMENT);
-		argv_array_pushf(env, GIT_PROTOCOL_ENVIRONMENT "=version=%d",
-				 version);
+		argv_array_pushf(env, GIT_PROTOCOL_ENVIRONMENT "=%s",
+				 version_advert->buf);
 	}
 
 	if (flags & CONNECT_IPV4) {
@@ -1164,7 +1160,7 @@ static void push_ssh_options(struct argv_array *args, struct argv_array *env,
 
 /* Prepare a child_process for use by Git's SSH-tunneled transport. */
 static void fill_ssh_args(struct child_process *conn, const char *ssh_host,
-			  const char *port, enum protocol_version version,
+			  const char *port, const struct strbuf *version_advert,
 			  int flags)
 {
 	const char *ssh;
@@ -1198,15 +1194,16 @@ static void fill_ssh_args(struct child_process *conn, const char *ssh_host,
 
 		argv_array_push(&detect.args, ssh);
 		argv_array_push(&detect.args, "-G");
-		push_ssh_options(&detect.args, &detect.env_array,
-				 VARIANT_SSH, port, version, flags);
+		push_ssh_options(&detect.args, &detect.env_array, VARIANT_SSH,
+				 port, version_advert, flags);
 		argv_array_push(&detect.args, ssh_host);
 
 		variant = run_command(&detect) ? VARIANT_SIMPLE : VARIANT_SSH;
 	}
 
 	argv_array_push(&conn->args, ssh);
-	push_ssh_options(&conn->args, &conn->env_array, variant, port, version, flags);
+	push_ssh_options(&conn->args, &conn->env_array, variant, port,
+			 version_advert, flags);
 	argv_array_push(&conn->args, ssh_host);
 }
 
@@ -1226,16 +1223,10 @@ struct child_process *git_connect(int fd[2], const char *url,
 {
 	char *hostandport, *path;
 	struct child_process *conn;
+	struct strbuf version_advert = STRBUF_INIT;
 	enum protocol protocol;
-	enum protocol_version version = get_protocol_version_config();
 
-	/*
-	 * NEEDSWORK: If we are trying to use protocol v2 and we are planning
-	 * to perform a push, then fallback to v0 since the client doesn't know
-	 * how to push yet using v2.
-	 */
-	if (version == protocol_v2 && !strcmp("git-receive-pack", prog))
-		version = protocol_v0;
+	get_client_protocol_version_advertisement(&version_advert);
 
 	/* Without this we cannot rely on waitpid() to tell
 	 * what happened to our children.
@@ -1250,7 +1241,8 @@ struct child_process *git_connect(int fd[2], const char *url,
 		printf("Diag: path=%s\n", path ? path : "NULL");
 		conn = NULL;
 	} else if (protocol == PROTO_GIT) {
-		conn = git_connect_git(fd, hostandport, path, prog, version, flags);
+		conn = git_connect_git(fd, hostandport, path, prog,
+				       &version_advert, flags);
 	} else {
 		struct strbuf cmd = STRBUF_INIT;
 		const char *const *var;
@@ -1293,13 +1285,13 @@ struct child_process *git_connect(int fd[2], const char *url,
 				strbuf_release(&cmd);
 				return NULL;
 			}
-			fill_ssh_args(conn, ssh_host, port, version, flags);
+			fill_ssh_args(conn, ssh_host, port, &version_advert,
+				      flags);
 		} else {
 			transport_check_allowed("file");
-			if (version > 0) {
-				argv_array_pushf(&conn->env_array, GIT_PROTOCOL_ENVIRONMENT "=version=%d",
-						 version);
-			}
+			argv_array_pushf(&conn->env_array,
+					 GIT_PROTOCOL_ENVIRONMENT "=%s",
+					 version_advert.buf);
 		}
 		argv_array_push(&conn->args, cmd.buf);
 
diff --git a/protocol.c b/protocol.c
index 5e636785d1..5664bd7a05 100644
--- a/protocol.c
+++ b/protocol.c
@@ -2,18 +2,43 @@
 #include "config.h"
 #include "protocol.h"
 
+static enum protocol_version *allowed_versions;
+static int nr_allowed_versions;
+static int alloc_allowed_versions;
+static int version_registration_locked = 0;
+
+static const char protocol_v0_string[] = "0";
+static const char protocol_v1_string[] = "1";
+static const char protocol_v2_string[] = "2";
+
 static enum protocol_version parse_protocol_version(const char *value)
 {
-	if (!strcmp(value, "0"))
+	if (!strcmp(value, protocol_v0_string))
 		return protocol_v0;
-	else if (!strcmp(value, "1"))
+	else if (!strcmp(value, protocol_v1_string))
 		return protocol_v1;
-	else if (!strcmp(value, "2"))
+	else if (!strcmp(value, protocol_v2_string))
 		return protocol_v2;
 	else
 		return protocol_unknown_version;
 }
 
+/* Return the text representation of a wire protocol version. */
+static const char *format_protocol_version(enum protocol_version version)
+{
+	switch (version) {
+	case protocol_v0:
+		return protocol_v0_string;
+	case protocol_v1:
+		return protocol_v1_string;
+	case protocol_v2:
+		return protocol_v2_string;
+	case protocol_unknown_version:
+		die(_("Unrecognized protocol version"));
+	}
+	die(_("Unrecognized protocol_version"));
+}
+
 enum protocol_version get_protocol_version_config(void)
 {
 	const char *value;
@@ -30,6 +55,85 @@ enum protocol_version get_protocol_version_config(void)
 	return protocol_v0;
 }
 
+void register_allowed_protocol_version(enum protocol_version version)
+{
+	if (version_registration_locked)
+		BUG("late attempt to register an allowed protocol version");
+
+	ALLOC_GROW(allowed_versions, nr_allowed_versions + 1,
+		   alloc_allowed_versions);
+	allowed_versions[nr_allowed_versions++] = version;
+}
+
+void register_allowed_protocol_versions_from_env(void)
+{
+	const char *git_protocol = getenv(GIT_PROTOCOL_ENVIRONMENT);
+	const char *version_str;
+	struct string_list version_list = STRING_LIST_INIT_DUP;
+	struct string_list_item *version;
+
+	if (!git_protocol)
+		return;
+
+	string_list_split(&version_list, git_protocol, ':', -1);
+	for_each_string_list_item(version, &version_list) {
+		if (skip_prefix(version->string, "version=", &version_str))
+			register_allowed_protocol_version(
+				parse_protocol_version(version_str));
+	}
+	string_list_clear(&version_list, 0);
+}
+
+static int is_allowed_protocol_version(enum protocol_version version)
+{
+	int i;
+	version_registration_locked = 1;
+	for (i = 0; i < nr_allowed_versions; i++)
+		if (version == allowed_versions[i])
+			return 1;
+	return 0;
+}
+
+void get_client_protocol_version_advertisement(struct strbuf *advert)
+{
+	int i, tmp_nr = nr_allowed_versions;
+	enum protocol_version *tmp_allowed_versions, config_version;
+	strbuf_reset(advert);
+
+	version_registration_locked = 1;
+
+	config_version = get_protocol_version_config();
+	if (config_version == protocol_v0) {
+		strbuf_addstr(advert, "version=0");
+		return;
+	}
+
+	if (tmp_nr > 0) {
+		ALLOC_ARRAY(tmp_allowed_versions, tmp_nr);
+		copy_array(tmp_allowed_versions, allowed_versions, tmp_nr,
+			   sizeof(enum protocol_version));
+	} else {
+		ALLOC_ARRAY(tmp_allowed_versions, 1);
+		tmp_nr = 1;
+		tmp_allowed_versions[0] = config_version;
+	}
+
+	if (tmp_allowed_versions[0] != config_version)
+		for (i = 1; i < nr_allowed_versions; i++)
+			if (tmp_allowed_versions[i] == config_version) {
+				SWAP(tmp_allowed_versions[0],
+				     tmp_allowed_versions[i]);
+			}
+
+	strbuf_addf(advert, "version=%s",
+		    format_protocol_version(tmp_allowed_versions[0]));
+	for (i = 1; i < tmp_nr; i++)
+		strbuf_addf(advert, ":version=%s",
+			    format_protocol_version(tmp_allowed_versions[i]));
+
+	free(tmp_allowed_versions);
+}
+
 enum protocol_version determine_protocol_version_server(void)
 {
 	const char *git_protocol = getenv(GIT_PROTOCOL_ENVIRONMENT);
@@ -38,9 +142,10 @@ enum protocol_version determine_protocol_version_server(void)
 	/*
 	 * Determine which protocol version the client has requested.  Since
 	 * multiple 'version' keys can be sent by the client, indicating that
-	 * the client is okay to speak any of them, select the greatest version
-	 * that the client has requested.  This is due to the assumption that
-	 * the most recent protocol version will be the most state-of-the-art.
+	 * the client is okay to speak any of them, select the first
+	 * recognizable version that the client has requested.  This is due to
+	 * the assumption that the protocol versions will be listed in
+	 * preference order.
 	 */
 	if (git_protocol) {
 		struct string_list list = STRING_LIST_INIT_DUP;
@@ -53,8 +158,11 @@ enum protocol_version determine_protocol_version_server(void)
 
 			if (skip_prefix(item->string, "version=", &value)) {
 				v = parse_protocol_version(value);
-				if (v > version)
+				if (v != protocol_unknown_version &&
+				    is_allowed_protocol_version(v)) {
 					version = v;
+					break;
+				}
 			}
 		}
 
diff --git a/protocol.h b/protocol.h
index 2ad35e433c..88330fd0ee 100644
--- a/protocol.h
+++ b/protocol.h
@@ -14,7 +14,24 @@ enum protocol_version {
  * 'protocol.version' config.  If unconfigured, a value of 'protocol_v0' is
  * returned.
  */
-extern enum protocol_version get_protocol_version_config(void);
+enum protocol_version get_protocol_version_config(void);
+
+/*
+ * Register an allowable protocol version for a given operation. Registration
+ * must occur before attempting to advertise a version to a server process.
+ */
+void register_allowed_protocol_version(enum protocol_version version);
+
+/*
+ * Register allowable protocol versions from the GIT_PROTOCOL environment var.
+ */
+void register_allowed_protocol_versions_from_env(void);
+
+/*
+ * Fill a strbuf with a version advertisement string suitable for use in the
+ * GIT_PROTOCOL environment variable or similar version negotiation field.
+ */
+void get_client_protocol_version_advertisement(struct strbuf *advert);
 
 /*
  * Used by a server to determine which protocol version should be used based on
@@ -23,12 +40,12 @@ extern enum protocol_version get_protocol_version_config(void);
  * request a particular protocol version, a default of 'protocol_v0' will be
  * used.
  */
-extern enum protocol_version determine_protocol_version_server(void);
+enum protocol_version determine_protocol_version_server(void);
 
 /*
  * Used by a client to determine which protocol version the server is speaking
  * based on the server's initial response.
  */
-extern enum protocol_version determine_protocol_version_client(const char *server_response);
+enum protocol_version determine_protocol_version_client(const char *server_response);
 
 #endif /* PROTOCOL_H */
diff --git a/remote-curl.c b/remote-curl.c
index fb28309e85..6ffefe5169 100644
--- a/remote-curl.c
+++ b/remote-curl.c
@@ -330,6 +330,19 @@ static int get_protocol_http_header(enum protocol_version version,
 	return 0;
 }
 
+static int get_client_protocol_http_header(const struct strbuf *version_advert,
+					   struct strbuf *header)
+{
+	if (version_advert->len > 0) {
+		strbuf_addf(header, GIT_PROTOCOL_HEADER ": %s",
+			    version_advert->buf);
+
+		return 1;
+	}
+
+	return 0;
+}
+
 static struct discovery *discover_refs(const char *service, int for_push)
 {
 	struct strbuf exp = STRBUF_INIT;
@@ -339,11 +352,11 @@ static struct discovery *discover_refs(const char *service, int for_push)
 	struct strbuf refs_url = STRBUF_INIT;
 	struct strbuf effective_url = STRBUF_INIT;
 	struct strbuf protocol_header = STRBUF_INIT;
+	struct strbuf version_advert = STRBUF_INIT;
 	struct string_list extra_headers = STRING_LIST_INIT_DUP;
 	struct discovery *last = last_discovery;
 	int http_ret, maybe_smart = 0;
 	struct http_get_options http_options;
-	enum protocol_version version = get_protocol_version_config();
 
 	if (last && !strcmp(service, last->service))
 		return last;
@@ -360,16 +373,10 @@ static struct discovery *discover_refs(const char *service, int for_push)
 		strbuf_addf(&refs_url, "service=%s", service);
 	}
 
-	/*
-	 * NEEDSWORK: If we are trying to use protocol v2 and we are planning
-	 * to perform a push, then fallback to v0 since the client doesn't know
-	 * how to push yet using v2.
-	 */
-	if (version == protocol_v2 && !strcmp("git-receive-pack", service))
-		version = protocol_v0;
+	get_client_protocol_version_advertisement(&version_advert);
 
 	/* Add the extra Git-Protocol header */
-	if (get_protocol_http_header(version, &protocol_header))
+	if (get_client_protocol_http_header(&version_advert, &protocol_header))
 		string_list_append(&extra_headers, protocol_header.buf);
 
 	memset(&http_options, 0, sizeof(http_options));
@@ -1327,6 +1334,8 @@ int cmd_main(int argc, const char **argv)
 	struct strbuf buf = STRBUF_INIT;
 	int nongit;
 
+	register_allowed_protocol_versions_from_env();
+
 	setup_git_directory_gently(&nongit);
 	if (argc < 2) {
 		error("remote-curl: usage: git remote-curl <remote> [<url>]");
diff --git a/t/t5551-http-fetch-smart.sh b/t/t5551-http-fetch-smart.sh
index 771f36f9ff..343bf3aafa 100755
--- a/t/t5551-http-fetch-smart.sh
+++ b/t/t5551-http-fetch-smart.sh
@@ -28,6 +28,7 @@ cat >exp <<EOF
 > Accept: */*
 > Accept-Encoding: ENCODINGS
 > Pragma: no-cache
+> Git-Protocol: version=0
 < HTTP/1.1 200 OK
 < Pragma: no-cache
 < Cache-Control: no-cache, max-age=0, must-revalidate
diff --git a/t/t5570-git-daemon.sh b/t/t5570-git-daemon.sh
index 7466aad111..d528e91630 100755
--- a/t/t5570-git-daemon.sh
+++ b/t/t5570-git-daemon.sh
@@ -186,7 +186,7 @@ test_expect_success 'hostname cannot break out of directory' '
 test_expect_success 'daemon log records all attributes' '
 	cat >expect <<-\EOF &&
 	Extended attribute "host": localhost
-	Extended attribute "protocol": version=1
+	Extended attribute "protocol": version=1:version=2:version=0
 	EOF
 	>daemon.log &&
 	GIT_OVERRIDE_VIRTUAL_HOST=localhost \
diff --git a/t/t5601-clone.sh b/t/t5601-clone.sh
index ddaa96ac4f..c4dbf1f779 100755
--- a/t/t5601-clone.sh
+++ b/t/t5601-clone.sh
@@ -346,7 +346,7 @@ expect_ssh () {
 
 test_expect_success 'clone myhost:src uses ssh' '
 	git clone myhost:src ssh-clone &&
-	expect_ssh myhost src
+	expect_ssh "-o SendEnv=GIT_PROTOCOL" myhost src
 '
 
 test_expect_success !MINGW,!CYGWIN 'clone local path foo:bar' '
@@ -357,12 +357,12 @@ test_expect_success !MINGW,!CYGWIN 'clone local path foo:bar' '
 
 test_expect_success 'bracketed hostnames are still ssh' '
 	git clone "[myhost:123]:src" ssh-bracket-clone &&
-	expect_ssh "-p 123" myhost src
+	expect_ssh "-o SendEnv=GIT_PROTOCOL -p 123" myhost src
 '
 
 test_expect_success 'OpenSSH variant passes -4' '
 	git clone -4 "[myhost:123]:src" ssh-ipv4-clone &&
-	expect_ssh "-4 -p 123" myhost src
+	expect_ssh "-o SendEnv=GIT_PROTOCOL -4 -p 123" myhost src
 '
 
 test_expect_success 'variant can be overridden' '
@@ -406,7 +406,7 @@ test_expect_success 'OpenSSH-like uplink is treated as ssh' '
 	GIT_SSH="$TRASH_DIRECTORY/uplink" &&
 	test_when_finished "GIT_SSH=\"\$TRASH_DIRECTORY/ssh\$X\"" &&
 	git clone "[myhost:123]:src" ssh-bracket-clone-sshlike-uplink &&
-	expect_ssh "-p 123" myhost src
+	expect_ssh "-o SendEnv=GIT_PROTOCOL -p 123" myhost src
 '
 
 test_expect_success 'plink is treated specially (as putty)' '
@@ -446,14 +446,14 @@ test_expect_success 'GIT_SSH_VARIANT overrides plink detection' '
 	copy_ssh_wrapper_as "$TRASH_DIRECTORY/plink" &&
 	GIT_SSH_VARIANT=ssh \
 	git clone "[myhost:123]:src" ssh-bracket-clone-variant-1 &&
-	expect_ssh "-p 123" myhost src
+	expect_ssh "-o SendEnv=GIT_PROTOCOL -p 123" myhost src
 '
 
 test_expect_success 'ssh.variant overrides plink detection' '
 	copy_ssh_wrapper_as "$TRASH_DIRECTORY/plink" &&
 	git -c ssh.variant=ssh \
 		clone "[myhost:123]:src" ssh-bracket-clone-variant-2 &&
-	expect_ssh "-p 123" myhost src
+	expect_ssh "-o SendEnv=GIT_PROTOCOL -p 123" myhost src
 '
 
 test_expect_success 'GIT_SSH_VARIANT overrides plink detection to plink' '
@@ -488,7 +488,7 @@ test_clone_url () {
 }
 
 test_expect_success !MINGW 'clone c:temp is ssl' '
-	test_clone_url c:temp c temp
+	test_clone_url c:temp "-o SendEnv=GIT_PROTOCOL" c temp
 '
 
 test_expect_success MINGW 'clone c:temp is dos drive' '
@@ -499,7 +499,7 @@ test_expect_success MINGW 'clone c:temp is dos drive' '
 for repo in rep rep/home/project 123
 do
 	test_expect_success "clone host:$repo" '
-		test_clone_url host:$repo host $repo
+		test_clone_url host:$repo "-o SendEnv=GIT_PROTOCOL" host $repo
 	'
 done
 
@@ -507,16 +507,16 @@ done
 for repo in rep rep/home/project 123
 do
 	test_expect_success "clone [::1]:$repo" '
-		test_clone_url [::1]:$repo ::1 "$repo"
+		test_clone_url [::1]:$repo "-o SendEnv=GIT_PROTOCOL" ::1 "$repo"
 	'
 done
 #home directory
 test_expect_success "clone host:/~repo" '
-	test_clone_url host:/~repo host "~repo"
+	test_clone_url host:/~repo "-o SendEnv=GIT_PROTOCOL" host "~repo"
 '
 
 test_expect_success "clone [::1]:/~repo" '
-	test_clone_url [::1]:/~repo ::1 "~repo"
+	test_clone_url [::1]:/~repo "-o SendEnv=GIT_PROTOCOL" ::1 "~repo"
 '
 
 # Corner cases
@@ -532,22 +532,22 @@ done
 for tcol in "" :
 do
 	test_expect_success "clone ssh://host.xz$tcol/home/user/repo" '
-		test_clone_url "ssh://host.xz$tcol/home/user/repo" host.xz /home/user/repo
+		test_clone_url "ssh://host.xz$tcol/home/user/repo" "-o SendEnv=GIT_PROTOCOL" host.xz /home/user/repo
 	'
 	# from home directory
 	test_expect_success "clone ssh://host.xz$tcol/~repo" '
-	test_clone_url "ssh://host.xz$tcol/~repo" host.xz "~repo"
+	test_clone_url "ssh://host.xz$tcol/~repo" "-o SendEnv=GIT_PROTOCOL" host.xz "~repo"
 '
 done
 
 # with port number
 test_expect_success 'clone ssh://host.xz:22/home/user/repo' '
-	test_clone_url "ssh://host.xz:22/home/user/repo" "-p 22 host.xz" "/home/user/repo"
+	test_clone_url "ssh://host.xz:22/home/user/repo" "-o SendEnv=GIT_PROTOCOL -p 22 host.xz" "/home/user/repo"
 '
 
 # from home directory with port number
 test_expect_success 'clone ssh://host.xz:22/~repo' '
-	test_clone_url "ssh://host.xz:22/~repo" "-p 22 host.xz" "~repo"
+	test_clone_url "ssh://host.xz:22/~repo" "-o SendEnv=GIT_PROTOCOL -p 22 host.xz" "~repo"
 '
 
 #IPv6
@@ -555,7 +555,7 @@ for tuah in ::1 [::1] [::1]: user@::1 user@[::1] user@[::1]: [user@::1] [user@::
 do
 	ehost=$(echo $tuah | sed -e "s/1]:/1]/" | tr -d "[]")
 	test_expect_success "clone ssh://$tuah/home/user/repo" "
-	  test_clone_url ssh://$tuah/home/user/repo $ehost /home/user/repo
+	  test_clone_url ssh://$tuah/home/user/repo '-o SendEnv=GIT_PROTOCOL' $ehost /home/user/repo
 	"
 done
 
@@ -564,7 +564,7 @@ for tuah in ::1 [::1] user@::1 user@[::1] [user@::1]
 do
 	euah=$(echo $tuah | tr -d "[]")
 	test_expect_success "clone ssh://$tuah/~repo" "
-	  test_clone_url ssh://$tuah/~repo $euah '~repo'
+	  test_clone_url ssh://$tuah/~repo '-o SendEnv=GIT_PROTOCOL' $euah '~repo'
 	"
 done
 
@@ -573,7 +573,7 @@ for tuah in [::1] user@[::1] [user@::1]
 do
 	euah=$(echo $tuah | tr -d "[]")
 	test_expect_success "clone ssh://$tuah:22/home/user/repo" "
-	  test_clone_url ssh://$tuah:22/home/user/repo '-p 22' $euah /home/user/repo
+	  test_clone_url ssh://$tuah:22/home/user/repo '-o SendEnv=GIT_PROTOCOL -p 22' $euah /home/user/repo
 	"
 done
 
@@ -582,7 +582,7 @@ for tuah in [::1] user@[::1] [user@::1]
 do
 	euah=$(echo $tuah | tr -d "[]")
 	test_expect_success "clone ssh://$tuah:22/~repo" "
-	  test_clone_url ssh://$tuah:22/~repo '-p 22' $euah '~repo'
+	  test_clone_url ssh://$tuah:22/~repo '-o SendEnv=GIT_PROTOCOL -p 22' $euah '~repo'
 	"
 done
 
diff --git a/t/t5700-protocol-v1.sh b/t/t5700-protocol-v1.sh
index ba86a44eb1..2e56c79233 100755
--- a/t/t5700-protocol-v1.sh
+++ b/t/t5700-protocol-v1.sh
@@ -26,7 +26,7 @@ test_expect_success 'clone with git:// using protocol v1' '
 	test_cmp expect actual &&
 
 	# Client requested to use protocol v1
-	grep "clone> .*\\\0\\\0version=1\\\0$" log &&
+	grep "clone> .*\\\0\\\0version=1.*\\\0$" log &&
 	# Server responded using protocol v1
 	grep "clone< version 1" log
 '
@@ -42,7 +42,7 @@ test_expect_success 'fetch with git:// using protocol v1' '
 	test_cmp expect actual &&
 
 	# Client requested to use protocol v1
-	grep "fetch> .*\\\0\\\0version=1\\\0$" log &&
+	grep "fetch> .*\\\0\\\0version=1.*\\\0$" log &&
 	# Server responded using protocol v1
 	grep "fetch< version 1" log
 '
@@ -56,7 +56,7 @@ test_expect_success 'pull with git:// using protocol v1' '
 	test_cmp expect actual &&
 
 	# Client requested to use protocol v1
-	grep "fetch> .*\\\0\\\0version=1\\\0$" log &&
+	grep "fetch> .*\\\0\\\0version=1.*\\\0$" log &&
 	# Server responded using protocol v1
 	grep "fetch< version 1" log
 '
@@ -74,7 +74,7 @@ test_expect_success 'push with git:// using protocol v1' '
 	test_cmp expect actual &&
 
 	# Client requested to use protocol v1
-	grep "push> .*\\\0\\\0version=1\\\0$" log &&
+	grep "push> .*\\\0\\\0version=1.*\\\0$" log &&
 	# Server responded using protocol v1
 	grep "push< version 1" log
 '
diff --git a/t/t5702-protocol-v2.sh b/t/t5702-protocol-v2.sh
index 3beeed4546..78c17c25e4 100755
--- a/t/t5702-protocol-v2.sh
+++ b/t/t5702-protocol-v2.sh
@@ -24,7 +24,7 @@ test_expect_success 'list refs with git:// using protocol v2' '
 		ls-remote --symref "$GIT_DAEMON_URL/parent" >actual &&
 
 	# Client requested to use protocol v2
-	grep "git> .*\\\0\\\0version=2\\\0$" log &&
+	grep "git> .*\\\0\\\0version=2.*\\\0$" log &&
 	# Server responded using protocol v2
 	grep "git< version 2" log &&
 
@@ -56,7 +56,7 @@ test_expect_success 'clone with git:// using protocol v2' '
 	test_cmp expect actual &&
 
 	# Client requested to use protocol v2
-	grep "clone> .*\\\0\\\0version=2\\\0$" log &&
+	grep "clone> .*\\\0\\\0version=2.*\\\0$" log &&
 	# Server responded using protocol v2
 	grep "clone< version 2" log
 '
@@ -74,7 +74,7 @@ test_expect_success 'fetch with git:// using protocol v2' '
 	test_cmp expect actual &&
 
 	# Client requested to use protocol v2
-	grep "fetch> .*\\\0\\\0version=2\\\0$" log &&
+	grep "fetch> .*\\\0\\\0version=2.*\\\0$" log &&
 	# Server responded using protocol v2
 	grep "fetch< version 2" log
 '
@@ -90,7 +90,7 @@ test_expect_success 'pull with git:// using protocol v2' '
 	test_cmp expect actual &&
 
 	# Client requested to use protocol v2
-	grep "fetch> .*\\\0\\\0version=2\\\0$" log &&
+	grep "fetch> .*\\\0\\\0version=2.*\\\0$" log &&
 	# Server responded using protocol v2
 	grep "fetch< version 2" log
 '
@@ -476,7 +476,7 @@ test_expect_success 'push with http:// and a config of v2 does not request v2' '
 	test_when_finished "rm -f log" &&
 	# Till v2 for push is designed, make sure that if a client has
 	# protocol.version configured to use v2, that the client instead falls
-	# back and uses v0.
+	# back to previous versions.
 
 	test_commit -C http_child three &&
 
@@ -489,10 +489,8 @@ test_expect_success 'push with http:// and a config of v2 does not request v2' '
 	git -C "$HTTPD_DOCUMENT_ROOT_PATH/http_parent" log -1 --format=%s client_branch >expect &&
 	test_cmp expect actual &&
 
-	# Client didnt request to use protocol v2
-	! grep "Git-Protocol: version=2" log &&
-	# Server didnt respond using protocol v2
-	! grep "git< version 2" log
+	# Server responded with version 1
+	grep "git< version 1" log
 '
 
 
diff --git a/transport-helper.c b/transport-helper.c
index 143ca008c8..ac1937f1e1 100644
--- a/transport-helper.c
+++ b/transport-helper.c
@@ -105,6 +105,7 @@ static struct child_process *get_helper(struct transport *transport)
 {
 	struct helper_data *data = transport->data;
 	struct strbuf buf = STRBUF_INIT;
+	struct strbuf version_advert = STRBUF_INIT;
 	struct child_process *helper;
 	int duped;
 	int code;
@@ -127,6 +128,11 @@ static struct child_process *get_helper(struct transport *transport)
 		argv_array_pushf(&helper->env_array, "%s=%s",
 				 GIT_DIR_ENVIRONMENT, get_git_dir());
 
+	get_client_protocol_version_advertisement(&version_advert);
+	if (version_advert.len > 0)
+		argv_array_pushf(&helper->env_array, "%s=%s",
+				 GIT_PROTOCOL_ENVIRONMENT, version_advert.buf);
+
 	code = start_command(helper);
 	if (code < 0 && errno == ENOENT)
 		die(_("unable to find remote helper for '%s'"), data->name);
-- 
2.19.1.1215.g8438c0b245-goog

Re: [PATCH v5 1/1] protocol: advertise multiple supported versions

[Ævar Arnfjörð Bjarmason]

On Fri, Nov 16 2018, Josh Steadmon wrote:

I started looking at this to address
https://public-inbox.org/git/nycvar.QRO.7.76.6.1812141318520.43@tvgsbejvaqbjf.bet/
and was about to send a re-roll of my own series, but then...

> Currently the client advertises that it supports the wire protocol
> version set in the protocol.version config. However, not all services
> support the same set of protocol versions. For example, git-receive-pack
> supports v1 and v0, but not v2. If a client connects to git-receive-pack
> and requests v2, it will instead be downgraded to v0. Other services,
> such as git-upload-archive, do not do any version negotiation checks.
>
> This patch creates a protocol version registry. Individual client and
> server programs register all the protocol versions they support prior to
> communicating with a remote instance. Versions should be listed in
> preference order; the version specified in protocol.version will
> automatically be moved to the front of the registry.
>
> The protocol version registry is passed to remote helpers via the
> GIT_PROTOCOL environment variable.
>
> Clients now advertise the full list of registered versions. Servers
> select the first allowed version from this advertisement.
>
> Additionally, remove special cases around advertising version=0.
> Previously we avoided adding version negotiation fields in server
> responses if it looked like the client wanted v0. However, including
> these fields does not change behavior, so it's better to have simpler
> code.

...this paragraph is new in your v5, from the cover letter: "Changes
from V4: remove special cases around advertising version=0". However as
seen in the code & tests:

> [...]
>  static void push_ssh_options(struct argv_array *args, struct argv_array *env,
>  			     enum ssh_variant variant, const char *port,
> -			     enum protocol_version version, int flags)
> +			     const struct strbuf *version_advert, int flags)
>  {
> -	if (variant == VARIANT_SSH &&
> -	    version > 0) {
> +	if (variant == VARIANT_SSH) {
>  		argv_array_push(args, "-o");
>  		argv_array_push(args, "SendEnv=" GIT_PROTOCOL_ENVIRONMENT);
> -		argv_array_pushf(env, GIT_PROTOCOL_ENVIRONMENT "=version=%d",
> -				 version);
> +		argv_array_pushf(env, GIT_PROTOCOL_ENVIRONMENT "=%s",
> +				 version_advert->buf);
>  	}
> [...]
> --- a/t/t5601-clone.sh
> +++ b/t/t5601-clone.sh
> @@ -346,7 +346,7 @@ expect_ssh () {
>
>  test_expect_success 'clone myhost:src uses ssh' '
>  	git clone myhost:src ssh-clone &&
> -	expect_ssh myhost src
> +	expect_ssh "-o SendEnv=GIT_PROTOCOL" myhost src
>  '
>
>  test_expect_success !MINGW,!CYGWIN 'clone local path foo:bar' '
> @@ -357,12 +357,12 @@ test_expect_success !MINGW,!CYGWIN 'clone local path foo:bar' '
>
>  test_expect_success 'bracketed hostnames are still ssh' '
>  	git clone "[myhost:123]:src" ssh-bracket-clone &&
> -	expect_ssh "-p 123" myhost src
> +	expect_ssh "-o SendEnv=GIT_PROTOCOL -p 123" myhost src
>  '
>
>  test_expect_success 'OpenSSH variant passes -4' '
>  	git clone -4 "[myhost:123]:src" ssh-ipv4-clone &&
> -	expect_ssh "-4 -p 123" myhost src
> +	expect_ssh "-o SendEnv=GIT_PROTOCOL -4 -p 123" myhost src
>  '
>
>  test_expect_success 'variant can be overridden' '
> @@ -406,7 +406,7 @@ test_expect_success 'OpenSSH-like uplink is treated as ssh' '
>  	GIT_SSH="$TRASH_DIRECTORY/uplink" &&
>  	test_when_finished "GIT_SSH=\"\$TRASH_DIRECTORY/ssh\$X\"" &&
>  	git clone "[myhost:123]:src" ssh-bracket-clone-sshlike-uplink &&
> -	expect_ssh "-p 123" myhost src
> +	expect_ssh "-o SendEnv=GIT_PROTOCOL -p 123" myhost src
>  '
>
>  test_expect_success 'plink is treated specially (as putty)' '
> @@ -446,14 +446,14 @@ test_expect_success 'GIT_SSH_VARIANT overrides plink detection' '
>  	copy_ssh_wrapper_as "$TRASH_DIRECTORY/plink" &&
>  	GIT_SSH_VARIANT=ssh \
>  	git clone "[myhost:123]:src" ssh-bracket-clone-variant-1 &&
> -	expect_ssh "-p 123" myhost src
> +	expect_ssh "-o SendEnv=GIT_PROTOCOL -p 123" myhost src
>  '
>
>  test_expect_success 'ssh.variant overrides plink detection' '
>  	copy_ssh_wrapper_as "$TRASH_DIRECTORY/plink" &&
>  	git -c ssh.variant=ssh \
>  		clone "[myhost:123]:src" ssh-bracket-clone-variant-2 &&
> -	expect_ssh "-p 123" myhost src
> +	expect_ssh "-o SendEnv=GIT_PROTOCOL -p 123" myhost src
>  '
>
>  test_expect_success 'GIT_SSH_VARIANT overrides plink detection to plink' '
> @@ -488,7 +488,7 @@ test_clone_url () {
>  }
>
>  test_expect_success !MINGW 'clone c:temp is ssl' '
> -	test_clone_url c:temp c temp
> +	test_clone_url c:temp "-o SendEnv=GIT_PROTOCOL" c temp
>  '
>
>  test_expect_success MINGW 'clone c:temp is dos drive' '
> @@ -499,7 +499,7 @@ test_expect_success MINGW 'clone c:temp is dos drive' '
>  for repo in rep rep/home/project 123
>  do
>  	test_expect_success "clone host:$repo" '
> -		test_clone_url host:$repo host $repo
> +		test_clone_url host:$repo "-o SendEnv=GIT_PROTOCOL" host $repo
>  	'
>  done
>
> @@ -507,16 +507,16 @@ done
>  for repo in rep rep/home/project 123
>  do
>  	test_expect_success "clone [::1]:$repo" '
> -		test_clone_url [::1]:$repo ::1 "$repo"
> +		test_clone_url [::1]:$repo "-o SendEnv=GIT_PROTOCOL" ::1 "$repo"
>  	'
>  done
>  #home directory
>  test_expect_success "clone host:/~repo" '
> -	test_clone_url host:/~repo host "~repo"
> +	test_clone_url host:/~repo "-o SendEnv=GIT_PROTOCOL" host "~repo"
>  '
>
>  test_expect_success "clone [::1]:/~repo" '
> -	test_clone_url [::1]:/~repo ::1 "~repo"
> +	test_clone_url [::1]:/~repo "-o SendEnv=GIT_PROTOCOL" ::1 "~repo"
>  '
>
>  # Corner cases
> @@ -532,22 +532,22 @@ done
>  for tcol in "" :
>  do
>  	test_expect_success "clone ssh://host.xz$tcol/home/user/repo" '
> -		test_clone_url "ssh://host.xz$tcol/home/user/repo" host.xz /home/user/repo
> +		test_clone_url "ssh://host.xz$tcol/home/user/repo" "-o SendEnv=GIT_PROTOCOL" host.xz /home/user/repo
>  	'
>  	# from home directory
>  	test_expect_success "clone ssh://host.xz$tcol/~repo" '
> -	test_clone_url "ssh://host.xz$tcol/~repo" host.xz "~repo"
> +	test_clone_url "ssh://host.xz$tcol/~repo" "-o SendEnv=GIT_PROTOCOL" host.xz "~repo"
>  '
>  done
>
>  # with port number
>  test_expect_success 'clone ssh://host.xz:22/home/user/repo' '
> -	test_clone_url "ssh://host.xz:22/home/user/repo" "-p 22 host.xz" "/home/user/repo"
> +	test_clone_url "ssh://host.xz:22/home/user/repo" "-o SendEnv=GIT_PROTOCOL -p 22 host.xz" "/home/user/repo"
>  '
>
>  # from home directory with port number
>  test_expect_success 'clone ssh://host.xz:22/~repo' '
> -	test_clone_url "ssh://host.xz:22/~repo" "-p 22 host.xz" "~repo"
> +	test_clone_url "ssh://host.xz:22/~repo" "-o SendEnv=GIT_PROTOCOL -p 22 host.xz" "~repo"
>  '
>
>  #IPv6
> @@ -555,7 +555,7 @@ for tuah in ::1 [::1] [::1]: user@::1 user@[::1] user@[::1]: [user@::1] [user@::
>  do
>  	ehost=$(echo $tuah | sed -e "s/1]:/1]/" | tr -d "[]")
>  	test_expect_success "clone ssh://$tuah/home/user/repo" "
> -	  test_clone_url ssh://$tuah/home/user/repo $ehost /home/user/repo
> +	  test_clone_url ssh://$tuah/home/user/repo '-o SendEnv=GIT_PROTOCOL' $ehost /home/user/repo
>  	"
>  done
>
> @@ -564,7 +564,7 @@ for tuah in ::1 [::1] user@::1 user@[::1] [user@::1]
>  do
>  	euah=$(echo $tuah | tr -d "[]")
>  	test_expect_success "clone ssh://$tuah/~repo" "
> -	  test_clone_url ssh://$tuah/~repo $euah '~repo'
> +	  test_clone_url ssh://$tuah/~repo '-o SendEnv=GIT_PROTOCOL' $euah '~repo'
>  	"
>  done
>
> @@ -573,7 +573,7 @@ for tuah in [::1] user@[::1] [user@::1]
>  do
>  	euah=$(echo $tuah | tr -d "[]")
>  	test_expect_success "clone ssh://$tuah:22/home/user/repo" "
> -	  test_clone_url ssh://$tuah:22/home/user/repo '-p 22' $euah /home/user/repo
> +	  test_clone_url ssh://$tuah:22/home/user/repo '-o SendEnv=GIT_PROTOCOL -p 22' $euah /home/user/repo
>  	"
>  done
>
> @@ -582,7 +582,7 @@ for tuah in [::1] user@[::1] [user@::1]
>  do
>  	euah=$(echo $tuah | tr -d "[]")
>  	test_expect_success "clone ssh://$tuah:22/~repo" "
> -	  test_clone_url ssh://$tuah:22/~repo '-p 22' $euah '~repo'
> +	  test_clone_url ssh://$tuah:22/~repo '-o SendEnv=GIT_PROTOCOL -p 22' $euah '~repo'
>  	"
>  done

...so now we're unconditionally going to SendEnv=GIT_PROTOCOL to "ssh"
invocations. I don't have an issue with this, but given the change in
the commit message this seems to have snuck under the radar. You're just
talking about always including the version in server responses, nothing
about the client always needing SendEnv=GIT_PROTOCOL now even with v0.

If the server always sends the version now, why don't you need to amend
the same t5400-send-pack.sh tests as in my "tests: mark & fix tests
broken under GIT_TEST_PROTOCOL_VERSION=1"? There's one that spews out
"version" there under my GIT_TEST_PROTOCOL_VERSION=1.

I was worried about this breaking GIT_SSH_COMMAND, but then I see due to
an interaction with picking "what ssh implementation?" we don't pass "-G
-o SendEnv=GIT_PROTOCOL" at all when I have a GIT_SSH_COMMAND, but *do*
pass it to my normal /usr/bin/ssh. Is this intended? Now if I have a
GIT_SSH_COMMAND that expects to wrap openssh I need to pass "-c
ssh.variant=ssh", because "-c ssh.variant=auto" will now omit these new
arguments.

Re: [PATCH v5 1/1] protocol: advertise multiple supported versions

[Josh Steadmon]

On 2018.12.14 21:20, Ævar Arnfjörð Bjarmason wrote:
> 
> On Fri, Nov 16 2018, Josh Steadmon wrote:
> 
> I started looking at this to address
> https://public-inbox.org/git/nycvar.QRO.7.76.6.1812141318520.43@tvgsbejvaqbjf.bet/
> and was about to send a re-roll of my own series, but then...
> 
> > Currently the client advertises that it supports the wire protocol
> > version set in the protocol.version config. However, not all services
> > support the same set of protocol versions. For example, git-receive-pack
> > supports v1 and v0, but not v2. If a client connects to git-receive-pack
> > and requests v2, it will instead be downgraded to v0. Other services,
> > such as git-upload-archive, do not do any version negotiation checks.
> >
> > This patch creates a protocol version registry. Individual client and
> > server programs register all the protocol versions they support prior to
> > communicating with a remote instance. Versions should be listed in
> > preference order; the version specified in protocol.version will
> > automatically be moved to the front of the registry.
> >
> > The protocol version registry is passed to remote helpers via the
> > GIT_PROTOCOL environment variable.
> >
> > Clients now advertise the full list of registered versions. Servers
> > select the first allowed version from this advertisement.
> >
> > Additionally, remove special cases around advertising version=0.
> > Previously we avoided adding version negotiation fields in server
> > responses if it looked like the client wanted v0. However, including
> > these fields does not change behavior, so it's better to have simpler
> > code.
> 
> ...this paragraph is new in your v5, from the cover letter: "Changes
> from V4: remove special cases around advertising version=0". However as
> seen in the code & tests:
> 
> > [...]
> >  static void push_ssh_options(struct argv_array *args, struct argv_array *env,
> >  			     enum ssh_variant variant, const char *port,
> > -			     enum protocol_version version, int flags)
> > +			     const struct strbuf *version_advert, int flags)
> >  {
> > -	if (variant == VARIANT_SSH &&
> > -	    version > 0) {
> > +	if (variant == VARIANT_SSH) {
> >  		argv_array_push(args, "-o");
> >  		argv_array_push(args, "SendEnv=" GIT_PROTOCOL_ENVIRONMENT);
> > -		argv_array_pushf(env, GIT_PROTOCOL_ENVIRONMENT "=version=%d",
> > -				 version);
> > +		argv_array_pushf(env, GIT_PROTOCOL_ENVIRONMENT "=%s",
> > +				 version_advert->buf);
> >  	}
> > [...]
> > --- a/t/t5601-clone.sh
> > +++ b/t/t5601-clone.sh
> > @@ -346,7 +346,7 @@ expect_ssh () {
> >
> >  test_expect_success 'clone myhost:src uses ssh' '
> >  	git clone myhost:src ssh-clone &&
> > -	expect_ssh myhost src
> > +	expect_ssh "-o SendEnv=GIT_PROTOCOL" myhost src
> >  '
> >
> >  test_expect_success !MINGW,!CYGWIN 'clone local path foo:bar' '
> > @@ -357,12 +357,12 @@ test_expect_success !MINGW,!CYGWIN 'clone local path foo:bar' '
> >
> >  test_expect_success 'bracketed hostnames are still ssh' '
> >  	git clone "[myhost:123]:src" ssh-bracket-clone &&
> > -	expect_ssh "-p 123" myhost src
> > +	expect_ssh "-o SendEnv=GIT_PROTOCOL -p 123" myhost src
> >  '
> >
> >  test_expect_success 'OpenSSH variant passes -4' '
> >  	git clone -4 "[myhost:123]:src" ssh-ipv4-clone &&
> > -	expect_ssh "-4 -p 123" myhost src
> > +	expect_ssh "-o SendEnv=GIT_PROTOCOL -4 -p 123" myhost src
> >  '
> >
> >  test_expect_success 'variant can be overridden' '
> > @@ -406,7 +406,7 @@ test_expect_success 'OpenSSH-like uplink is treated as ssh' '
> >  	GIT_SSH="$TRASH_DIRECTORY/uplink" &&
> >  	test_when_finished "GIT_SSH=\"\$TRASH_DIRECTORY/ssh\$X\"" &&
> >  	git clone "[myhost:123]:src" ssh-bracket-clone-sshlike-uplink &&
> > -	expect_ssh "-p 123" myhost src
> > +	expect_ssh "-o SendEnv=GIT_PROTOCOL -p 123" myhost src
> >  '
> >
> >  test_expect_success 'plink is treated specially (as putty)' '
> > @@ -446,14 +446,14 @@ test_expect_success 'GIT_SSH_VARIANT overrides plink detection' '
> >  	copy_ssh_wrapper_as "$TRASH_DIRECTORY/plink" &&
> >  	GIT_SSH_VARIANT=ssh \
> >  	git clone "[myhost:123]:src" ssh-bracket-clone-variant-1 &&
> > -	expect_ssh "-p 123" myhost src
> > +	expect_ssh "-o SendEnv=GIT_PROTOCOL -p 123" myhost src
> >  '
> >
> >  test_expect_success 'ssh.variant overrides plink detection' '
> >  	copy_ssh_wrapper_as "$TRASH_DIRECTORY/plink" &&
> >  	git -c ssh.variant=ssh \
> >  		clone "[myhost:123]:src" ssh-bracket-clone-variant-2 &&
> > -	expect_ssh "-p 123" myhost src
> > +	expect_ssh "-o SendEnv=GIT_PROTOCOL -p 123" myhost src
> >  '
> >
> >  test_expect_success 'GIT_SSH_VARIANT overrides plink detection to plink' '
> > @@ -488,7 +488,7 @@ test_clone_url () {
> >  }
> >
> >  test_expect_success !MINGW 'clone c:temp is ssl' '
> > -	test_clone_url c:temp c temp
> > +	test_clone_url c:temp "-o SendEnv=GIT_PROTOCOL" c temp
> >  '
> >
> >  test_expect_success MINGW 'clone c:temp is dos drive' '
> > @@ -499,7 +499,7 @@ test_expect_success MINGW 'clone c:temp is dos drive' '
> >  for repo in rep rep/home/project 123
> >  do
> >  	test_expect_success "clone host:$repo" '
> > -		test_clone_url host:$repo host $repo
> > +		test_clone_url host:$repo "-o SendEnv=GIT_PROTOCOL" host $repo
> >  	'
> >  done
> >
> > @@ -507,16 +507,16 @@ done
> >  for repo in rep rep/home/project 123
> >  do
> >  	test_expect_success "clone [::1]:$repo" '
> > -		test_clone_url [::1]:$repo ::1 "$repo"
> > +		test_clone_url [::1]:$repo "-o SendEnv=GIT_PROTOCOL" ::1 "$repo"
> >  	'
> >  done
> >  #home directory
> >  test_expect_success "clone host:/~repo" '
> > -	test_clone_url host:/~repo host "~repo"
> > +	test_clone_url host:/~repo "-o SendEnv=GIT_PROTOCOL" host "~repo"
> >  '
> >
> >  test_expect_success "clone [::1]:/~repo" '
> > -	test_clone_url [::1]:/~repo ::1 "~repo"
> > +	test_clone_url [::1]:/~repo "-o SendEnv=GIT_PROTOCOL" ::1 "~repo"
> >  '
> >
> >  # Corner cases
> > @@ -532,22 +532,22 @@ done
> >  for tcol in "" :
> >  do
> >  	test_expect_success "clone ssh://host.xz$tcol/home/user/repo" '
> > -		test_clone_url "ssh://host.xz$tcol/home/user/repo" host.xz /home/user/repo
> > +		test_clone_url "ssh://host.xz$tcol/home/user/repo" "-o SendEnv=GIT_PROTOCOL" host.xz /home/user/repo
> >  	'
> >  	# from home directory
> >  	test_expect_success "clone ssh://host.xz$tcol/~repo" '
> > -	test_clone_url "ssh://host.xz$tcol/~repo" host.xz "~repo"
> > +	test_clone_url "ssh://host.xz$tcol/~repo" "-o SendEnv=GIT_PROTOCOL" host.xz "~repo"
> >  '
> >  done
> >
> >  # with port number
> >  test_expect_success 'clone ssh://host.xz:22/home/user/repo' '
> > -	test_clone_url "ssh://host.xz:22/home/user/repo" "-p 22 host.xz" "/home/user/repo"
> > +	test_clone_url "ssh://host.xz:22/home/user/repo" "-o SendEnv=GIT_PROTOCOL -p 22 host.xz" "/home/user/repo"
> >  '
> >
> >  # from home directory with port number
> >  test_expect_success 'clone ssh://host.xz:22/~repo' '
> > -	test_clone_url "ssh://host.xz:22/~repo" "-p 22 host.xz" "~repo"
> > +	test_clone_url "ssh://host.xz:22/~repo" "-o SendEnv=GIT_PROTOCOL -p 22 host.xz" "~repo"
> >  '
> >
> >  #IPv6
> > @@ -555,7 +555,7 @@ for tuah in ::1 [::1] [::1]: user@::1 user@[::1] user@[::1]: [user@::1] [user@::
> >  do
> >  	ehost=$(echo $tuah | sed -e "s/1]:/1]/" | tr -d "[]")
> >  	test_expect_success "clone ssh://$tuah/home/user/repo" "
> > -	  test_clone_url ssh://$tuah/home/user/repo $ehost /home/user/repo
> > +	  test_clone_url ssh://$tuah/home/user/repo '-o SendEnv=GIT_PROTOCOL' $ehost /home/user/repo
> >  	"
> >  done
> >
> > @@ -564,7 +564,7 @@ for tuah in ::1 [::1] user@::1 user@[::1] [user@::1]
> >  do
> >  	euah=$(echo $tuah | tr -d "[]")
> >  	test_expect_success "clone ssh://$tuah/~repo" "
> > -	  test_clone_url ssh://$tuah/~repo $euah '~repo'
> > +	  test_clone_url ssh://$tuah/~repo '-o SendEnv=GIT_PROTOCOL' $euah '~repo'
> >  	"
> >  done
> >
> > @@ -573,7 +573,7 @@ for tuah in [::1] user@[::1] [user@::1]
> >  do
> >  	euah=$(echo $tuah | tr -d "[]")
> >  	test_expect_success "clone ssh://$tuah:22/home/user/repo" "
> > -	  test_clone_url ssh://$tuah:22/home/user/repo '-p 22' $euah /home/user/repo
> > +	  test_clone_url ssh://$tuah:22/home/user/repo '-o SendEnv=GIT_PROTOCOL -p 22' $euah /home/user/repo
> >  	"
> >  done
> >
> > @@ -582,7 +582,7 @@ for tuah in [::1] user@[::1] [user@::1]
> >  do
> >  	euah=$(echo $tuah | tr -d "[]")
> >  	test_expect_success "clone ssh://$tuah:22/~repo" "
> > -	  test_clone_url ssh://$tuah:22/~repo '-p 22' $euah '~repo'
> > +	  test_clone_url ssh://$tuah:22/~repo '-o SendEnv=GIT_PROTOCOL -p 22' $euah '~repo'
> >  	"
> >  done
> 
> ...so now we're unconditionally going to SendEnv=GIT_PROTOCOL to "ssh"
> invocations. I don't have an issue with this, but given the change in
> the commit message this seems to have snuck under the radar. You're just
> talking about always including the version in server responses, nothing
> about the client always needing SendEnv=GIT_PROTOCOL now even with v0.

Ack. I'll make sure that V6 calls this out in the commit message.


> If the server always sends the version now, why don't you need to amend
> the same t5400-send-pack.sh tests as in my "tests: mark & fix tests
> broken under GIT_TEST_PROTOCOL_VERSION=1"? There's one that spews out
> "version" there under my GIT_TEST_PROTOCOL_VERSION=1.

Sorry if I'm being dense, but can you point out more specifically what
is wrong here? I don't see anything in t5400 that would be affected by
this; the final test case ("receive-pack de-dupes .have lines") is the
only one that does any tracing, and it strips out everything that's not
a ref advertisement before checking the output. Sorry if I'm missing
something obvious.


> I was worried about this breaking GIT_SSH_COMMAND, but then I see due to
> an interaction with picking "what ssh implementation?" we don't pass "-G
> -o SendEnv=GIT_PROTOCOL" at all when I have a GIT_SSH_COMMAND, but *do*
> pass it to my normal /usr/bin/ssh. Is this intended? Now if I have a
> GIT_SSH_COMMAND that expects to wrap openssh I need to pass "-c
> ssh.variant=ssh", because "-c ssh.variant=auto" will now omit these new
> arguments.

I am not an expert on this part of the code, but it seems to be
intended. Later on in the function, there are BUG()s that state that
VARIANT_AUTO should not be passed to the push_ssh_options() function.
And this only changes the behavior when version=0. For protocol versions
1 & 2, VARIANT_AUTO never had SendEnv=GIT_PROTOCOL added to the command
line. Again, sorry if I'm missing some implication here.


Thanks,
Josh

Re: [PATCH v5 1/1] protocol: advertise multiple supported versions

[Ævar Arnfjörð Bjarmason]

On Fri, Dec 14 2018, Josh Steadmon wrote:

> On 2018.12.14 21:20, Ævar Arnfjörð Bjarmason wrote:
>>
>> On Fri, Nov 16 2018, Josh Steadmon wrote:
>>
>> I started looking at this to address
>> https://public-inbox.org/git/nycvar.QRO.7.76.6.1812141318520.43@tvgsbejvaqbjf.bet/
>> and was about to send a re-roll of my own series, but then...
>>
>> > Currently the client advertises that it supports the wire protocol
>> > version set in the protocol.version config. However, not all services
>> > support the same set of protocol versions. For example, git-receive-pack
>> > supports v1 and v0, but not v2. If a client connects to git-receive-pack
>> > and requests v2, it will instead be downgraded to v0. Other services,
>> > such as git-upload-archive, do not do any version negotiation checks.
>> >
>> > This patch creates a protocol version registry. Individual client and
>> > server programs register all the protocol versions they support prior to
>> > communicating with a remote instance. Versions should be listed in
>> > preference order; the version specified in protocol.version will
>> > automatically be moved to the front of the registry.
>> >
>> > The protocol version registry is passed to remote helpers via the
>> > GIT_PROTOCOL environment variable.
>> >
>> > Clients now advertise the full list of registered versions. Servers
>> > select the first allowed version from this advertisement.
>> >
>> > Additionally, remove special cases around advertising version=0.
>> > Previously we avoided adding version negotiation fields in server
>> > responses if it looked like the client wanted v0. However, including
>> > these fields does not change behavior, so it's better to have simpler
>> > code.
>>
>> ...this paragraph is new in your v5, from the cover letter: "Changes
>> from V4: remove special cases around advertising version=0". However as
>> seen in the code & tests:
>>
>> > [...]
>> >  static void push_ssh_options(struct argv_array *args, struct argv_array *env,
>> >  			     enum ssh_variant variant, const char *port,
>> > -			     enum protocol_version version, int flags)
>> > +			     const struct strbuf *version_advert, int flags)
>> >  {
>> > -	if (variant == VARIANT_SSH &&
>> > -	    version > 0) {
>> > +	if (variant == VARIANT_SSH) {
>> >  		argv_array_push(args, "-o");
>> >  		argv_array_push(args, "SendEnv=" GIT_PROTOCOL_ENVIRONMENT);
>> > -		argv_array_pushf(env, GIT_PROTOCOL_ENVIRONMENT "=version=%d",
>> > -				 version);
>> > +		argv_array_pushf(env, GIT_PROTOCOL_ENVIRONMENT "=%s",
>> > +				 version_advert->buf);
>> >  	}
>> > [...]
>> > --- a/t/t5601-clone.sh
>> > +++ b/t/t5601-clone.sh
>> > @@ -346,7 +346,7 @@ expect_ssh () {
>> >
>> >  test_expect_success 'clone myhost:src uses ssh' '
>> >  	git clone myhost:src ssh-clone &&
>> > -	expect_ssh myhost src
>> > +	expect_ssh "-o SendEnv=GIT_PROTOCOL" myhost src
>> >  '
>> >
>> >  test_expect_success !MINGW,!CYGWIN 'clone local path foo:bar' '
>> > @@ -357,12 +357,12 @@ test_expect_success !MINGW,!CYGWIN 'clone local path foo:bar' '
>> >
>> >  test_expect_success 'bracketed hostnames are still ssh' '
>> >  	git clone "[myhost:123]:src" ssh-bracket-clone &&
>> > -	expect_ssh "-p 123" myhost src
>> > +	expect_ssh "-o SendEnv=GIT_PROTOCOL -p 123" myhost src
>> >  '
>> >
>> >  test_expect_success 'OpenSSH variant passes -4' '
>> >  	git clone -4 "[myhost:123]:src" ssh-ipv4-clone &&
>> > -	expect_ssh "-4 -p 123" myhost src
>> > +	expect_ssh "-o SendEnv=GIT_PROTOCOL -4 -p 123" myhost src
>> >  '
>> >
>> >  test_expect_success 'variant can be overridden' '
>> > @@ -406,7 +406,7 @@ test_expect_success 'OpenSSH-like uplink is treated as ssh' '
>> >  	GIT_SSH="$TRASH_DIRECTORY/uplink" &&
>> >  	test_when_finished "GIT_SSH=\"\$TRASH_DIRECTORY/ssh\$X\"" &&
>> >  	git clone "[myhost:123]:src" ssh-bracket-clone-sshlike-uplink &&
>> > -	expect_ssh "-p 123" myhost src
>> > +	expect_ssh "-o SendEnv=GIT_PROTOCOL -p 123" myhost src
>> >  '
>> >
>> >  test_expect_success 'plink is treated specially (as putty)' '
>> > @@ -446,14 +446,14 @@ test_expect_success 'GIT_SSH_VARIANT overrides plink detection' '
>> >  	copy_ssh_wrapper_as "$TRASH_DIRECTORY/plink" &&
>> >  	GIT_SSH_VARIANT=ssh \
>> >  	git clone "[myhost:123]:src" ssh-bracket-clone-variant-1 &&
>> > -	expect_ssh "-p 123" myhost src
>> > +	expect_ssh "-o SendEnv=GIT_PROTOCOL -p 123" myhost src
>> >  '
>> >
>> >  test_expect_success 'ssh.variant overrides plink detection' '
>> >  	copy_ssh_wrapper_as "$TRASH_DIRECTORY/plink" &&
>> >  	git -c ssh.variant=ssh \
>> >  		clone "[myhost:123]:src" ssh-bracket-clone-variant-2 &&
>> > -	expect_ssh "-p 123" myhost src
>> > +	expect_ssh "-o SendEnv=GIT_PROTOCOL -p 123" myhost src
>> >  '
>> >
>> >  test_expect_success 'GIT_SSH_VARIANT overrides plink detection to plink' '
>> > @@ -488,7 +488,7 @@ test_clone_url () {
>> >  }
>> >
>> >  test_expect_success !MINGW 'clone c:temp is ssl' '
>> > -	test_clone_url c:temp c temp
>> > +	test_clone_url c:temp "-o SendEnv=GIT_PROTOCOL" c temp
>> >  '
>> >
>> >  test_expect_success MINGW 'clone c:temp is dos drive' '
>> > @@ -499,7 +499,7 @@ test_expect_success MINGW 'clone c:temp is dos drive' '
>> >  for repo in rep rep/home/project 123
>> >  do
>> >  	test_expect_success "clone host:$repo" '
>> > -		test_clone_url host:$repo host $repo
>> > +		test_clone_url host:$repo "-o SendEnv=GIT_PROTOCOL" host $repo
>> >  	'
>> >  done
>> >
>> > @@ -507,16 +507,16 @@ done
>> >  for repo in rep rep/home/project 123
>> >  do
>> >  	test_expect_success "clone [::1]:$repo" '
>> > -		test_clone_url [::1]:$repo ::1 "$repo"
>> > +		test_clone_url [::1]:$repo "-o SendEnv=GIT_PROTOCOL" ::1 "$repo"
>> >  	'
>> >  done
>> >  #home directory
>> >  test_expect_success "clone host:/~repo" '
>> > -	test_clone_url host:/~repo host "~repo"
>> > +	test_clone_url host:/~repo "-o SendEnv=GIT_PROTOCOL" host "~repo"
>> >  '
>> >
>> >  test_expect_success "clone [::1]:/~repo" '
>> > -	test_clone_url [::1]:/~repo ::1 "~repo"
>> > +	test_clone_url [::1]:/~repo "-o SendEnv=GIT_PROTOCOL" ::1 "~repo"
>> >  '
>> >
>> >  # Corner cases
>> > @@ -532,22 +532,22 @@ done
>> >  for tcol in "" :
>> >  do
>> >  	test_expect_success "clone ssh://host.xz$tcol/home/user/repo" '
>> > -		test_clone_url "ssh://host.xz$tcol/home/user/repo" host.xz /home/user/repo
>> > +		test_clone_url "ssh://host.xz$tcol/home/user/repo" "-o SendEnv=GIT_PROTOCOL" host.xz /home/user/repo
>> >  	'
>> >  	# from home directory
>> >  	test_expect_success "clone ssh://host.xz$tcol/~repo" '
>> > -	test_clone_url "ssh://host.xz$tcol/~repo" host.xz "~repo"
>> > +	test_clone_url "ssh://host.xz$tcol/~repo" "-o SendEnv=GIT_PROTOCOL" host.xz "~repo"
>> >  '
>> >  done
>> >
>> >  # with port number
>> >  test_expect_success 'clone ssh://host.xz:22/home/user/repo' '
>> > -	test_clone_url "ssh://host.xz:22/home/user/repo" "-p 22 host.xz" "/home/user/repo"
>> > +	test_clone_url "ssh://host.xz:22/home/user/repo" "-o SendEnv=GIT_PROTOCOL -p 22 host.xz" "/home/user/repo"
>> >  '
>> >
>> >  # from home directory with port number
>> >  test_expect_success 'clone ssh://host.xz:22/~repo' '
>> > -	test_clone_url "ssh://host.xz:22/~repo" "-p 22 host.xz" "~repo"
>> > +	test_clone_url "ssh://host.xz:22/~repo" "-o SendEnv=GIT_PROTOCOL -p 22 host.xz" "~repo"
>> >  '
>> >
>> >  #IPv6
>> > @@ -555,7 +555,7 @@ for tuah in ::1 [::1] [::1]: user@::1 user@[::1] user@[::1]: [user@::1] [user@::
>> >  do
>> >  	ehost=$(echo $tuah | sed -e "s/1]:/1]/" | tr -d "[]")
>> >  	test_expect_success "clone ssh://$tuah/home/user/repo" "
>> > -	  test_clone_url ssh://$tuah/home/user/repo $ehost /home/user/repo
>> > +	  test_clone_url ssh://$tuah/home/user/repo '-o SendEnv=GIT_PROTOCOL' $ehost /home/user/repo
>> >  	"
>> >  done
>> >
>> > @@ -564,7 +564,7 @@ for tuah in ::1 [::1] user@::1 user@[::1] [user@::1]
>> >  do
>> >  	euah=$(echo $tuah | tr -d "[]")
>> >  	test_expect_success "clone ssh://$tuah/~repo" "
>> > -	  test_clone_url ssh://$tuah/~repo $euah '~repo'
>> > +	  test_clone_url ssh://$tuah/~repo '-o SendEnv=GIT_PROTOCOL' $euah '~repo'
>> >  	"
>> >  done
>> >
>> > @@ -573,7 +573,7 @@ for tuah in [::1] user@[::1] [user@::1]
>> >  do
>> >  	euah=$(echo $tuah | tr -d "[]")
>> >  	test_expect_success "clone ssh://$tuah:22/home/user/repo" "
>> > -	  test_clone_url ssh://$tuah:22/home/user/repo '-p 22' $euah /home/user/repo
>> > +	  test_clone_url ssh://$tuah:22/home/user/repo '-o SendEnv=GIT_PROTOCOL -p 22' $euah /home/user/repo
>> >  	"
>> >  done
>> >
>> > @@ -582,7 +582,7 @@ for tuah in [::1] user@[::1] [user@::1]
>> >  do
>> >  	euah=$(echo $tuah | tr -d "[]")
>> >  	test_expect_success "clone ssh://$tuah:22/~repo" "
>> > -	  test_clone_url ssh://$tuah:22/~repo '-p 22' $euah '~repo'
>> > +	  test_clone_url ssh://$tuah:22/~repo '-o SendEnv=GIT_PROTOCOL -p 22' $euah '~repo'
>> >  	"
>> >  done
>>
>> ...so now we're unconditionally going to SendEnv=GIT_PROTOCOL to "ssh"
>> invocations. I don't have an issue with this, but given the change in
>> the commit message this seems to have snuck under the radar. You're just
>> talking about always including the version in server responses, nothing
>> about the client always needing SendEnv=GIT_PROTOCOL now even with v0.
>
> Ack. I'll make sure that V6 calls this out in the commit message.
>
>
>> If the server always sends the version now, why don't you need to amend
>> the same t5400-send-pack.sh tests as in my "tests: mark & fix tests
>> broken under GIT_TEST_PROTOCOL_VERSION=1"? There's one that spews out
>> "version" there under my GIT_TEST_PROTOCOL_VERSION=1.
>
> Sorry if I'm being dense, but can you point out more specifically what
> is wrong here? I don't see anything in t5400 that would be affected by
> this; the final test case ("receive-pack de-dupes .have lines") is the
> only one that does any tracing, and it strips out everything that's not
> a ref advertisement before checking the output. Sorry if I'm missing
> something obvious.

I think I'm just misunderstanding this bit:

    Additionally, remove special cases around advertising version=0.
    Previously we avoided adding version negotiation fields in server
    responses if it looked like the client wanted v0. However, including
    these fields does not change behavior, so it's better to have
    simpler code.

I meant that if you pick the series I have up to "tests: add a special
setup that sets protocol.version", which is at c6f33984fc in a WIP
branch in github.com/avar/git.git and run:

    $ GIT_TEST_PROTOCOL_VERSION=1 ./t5400-send-pack.sh -v -i -x

It'll fail with:

    + test_cmp expect refs
    + diff -u expect refs
    --- expect      2018-12-14 22:26:52.485411992 +0000
    +++ refs        2018-12-14 22:26:52.713412148 +0000
    @@ -1,3 +1,4 @@
    +version 1
     5285e1710002a06a379056b0d21357a999f3c642 refs/heads/master
     5285e1710002a06a379056b0d21357a999f3c642 refs/remotes/origin/HEAD
     5285e1710002a06a379056b0d21357a999f3c642 refs/remotes/origin/master
    error: last command exited with $?=1
    not ok 16 - receive-pack de-dupes .have lines

And I read your commit message to mean "v0 clients also support v1
responses with the version in them, so let's just always send it". But I
think this is my confusion (but I still don't know what it *really*
means).

>> I was worried about this breaking GIT_SSH_COMMAND, but then I see due to
>> an interaction with picking "what ssh implementation?" we don't pass "-G
>> -o SendEnv=GIT_PROTOCOL" at all when I have a GIT_SSH_COMMAND, but *do*
>> pass it to my normal /usr/bin/ssh. Is this intended? Now if I have a
>> GIT_SSH_COMMAND that expects to wrap openssh I need to pass "-c
>> ssh.variant=ssh", because "-c ssh.variant=auto" will now omit these new
>> arguments.
>
> I am not an expert on this part of the code, but it seems to be
> intended. Later on in the function, there are BUG()s that state that
> VARIANT_AUTO should not be passed to the push_ssh_options() function.
> And this only changes the behavior when version=0. For protocol versions
> 1 & 2, VARIANT_AUTO never had SendEnv=GIT_PROTOCOL added to the command
> line. Again, sorry if I'm missing some implication here.

I'm wondering if we need to worry about some new odd interactions
between client/servers when using GIT_SSH* wrappers, maybe not, but
e.g. as opposed to 0da0e49ba1 ("ssh: 'auto' variant to select between
'ssh' and 'simple'", 2017-11-20) which noted a similar change had been
tested at Google internally (and I read it as Google using GIT_SSH* on
workstations) your commit doesn't make any mention of this case being
tested / considered.

So do we need to worry about some new interaction here? Maybe not. Just
something for people more experienced with the ssh integration to chime
in on.

Re: [PATCH v5 1/1] protocol: advertise multiple supported versions

[Josh Steadmon]

On 2018.12.14 23:39, Ævar Arnfjörð Bjarmason wrote:
> 
> On Fri, Dec 14 2018, Josh Steadmon wrote:
> 
> > On 2018.12.14 21:20, Ævar Arnfjörð Bjarmason wrote:
> >>
> >> On Fri, Nov 16 2018, Josh Steadmon wrote:
> >>
> >> I started looking at this to address
> >> https://public-inbox.org/git/nycvar.QRO.7.76.6.1812141318520.43@tvgsbejvaqbjf.bet/
> >> and was about to send a re-roll of my own series, but then...
> >>
> >> > Currently the client advertises that it supports the wire protocol
> >> > version set in the protocol.version config. However, not all services
> >> > support the same set of protocol versions. For example, git-receive-pack
> >> > supports v1 and v0, but not v2. If a client connects to git-receive-pack
> >> > and requests v2, it will instead be downgraded to v0. Other services,
> >> > such as git-upload-archive, do not do any version negotiation checks.
> >> >
> >> > This patch creates a protocol version registry. Individual client and
> >> > server programs register all the protocol versions they support prior to
> >> > communicating with a remote instance. Versions should be listed in
> >> > preference order; the version specified in protocol.version will
> >> > automatically be moved to the front of the registry.
> >> >
> >> > The protocol version registry is passed to remote helpers via the
> >> > GIT_PROTOCOL environment variable.
> >> >
> >> > Clients now advertise the full list of registered versions. Servers
> >> > select the first allowed version from this advertisement.
> >> >
> >> > Additionally, remove special cases around advertising version=0.
> >> > Previously we avoided adding version negotiation fields in server
> >> > responses if it looked like the client wanted v0. However, including
> >> > these fields does not change behavior, so it's better to have simpler
> >> > code.
> >>
> >> ...this paragraph is new in your v5, from the cover letter: "Changes
> >> from V4: remove special cases around advertising version=0". However as
> >> seen in the code & tests:
> >>
> >> > [...]
> >> >  static void push_ssh_options(struct argv_array *args, struct argv_array *env,
> >> >  			     enum ssh_variant variant, const char *port,
> >> > -			     enum protocol_version version, int flags)
> >> > +			     const struct strbuf *version_advert, int flags)
> >> >  {
> >> > -	if (variant == VARIANT_SSH &&
> >> > -	    version > 0) {
> >> > +	if (variant == VARIANT_SSH) {
> >> >  		argv_array_push(args, "-o");
> >> >  		argv_array_push(args, "SendEnv=" GIT_PROTOCOL_ENVIRONMENT);
> >> > -		argv_array_pushf(env, GIT_PROTOCOL_ENVIRONMENT "=version=%d",
> >> > -				 version);
> >> > +		argv_array_pushf(env, GIT_PROTOCOL_ENVIRONMENT "=%s",
> >> > +				 version_advert->buf);
> >> >  	}
> >> > [...]
> >> > --- a/t/t5601-clone.sh
> >> > +++ b/t/t5601-clone.sh
> >> > @@ -346,7 +346,7 @@ expect_ssh () {
> >> >
> >> >  test_expect_success 'clone myhost:src uses ssh' '
> >> >  	git clone myhost:src ssh-clone &&
> >> > -	expect_ssh myhost src
> >> > +	expect_ssh "-o SendEnv=GIT_PROTOCOL" myhost src
> >> >  '
> >> >
> >> >  test_expect_success !MINGW,!CYGWIN 'clone local path foo:bar' '
> >> > @@ -357,12 +357,12 @@ test_expect_success !MINGW,!CYGWIN 'clone local path foo:bar' '
> >> >
> >> >  test_expect_success 'bracketed hostnames are still ssh' '
> >> >  	git clone "[myhost:123]:src" ssh-bracket-clone &&
> >> > -	expect_ssh "-p 123" myhost src
> >> > +	expect_ssh "-o SendEnv=GIT_PROTOCOL -p 123" myhost src
> >> >  '
> >> >
> >> >  test_expect_success 'OpenSSH variant passes -4' '
> >> >  	git clone -4 "[myhost:123]:src" ssh-ipv4-clone &&
> >> > -	expect_ssh "-4 -p 123" myhost src
> >> > +	expect_ssh "-o SendEnv=GIT_PROTOCOL -4 -p 123" myhost src
> >> >  '
> >> >
> >> >  test_expect_success 'variant can be overridden' '
> >> > @@ -406,7 +406,7 @@ test_expect_success 'OpenSSH-like uplink is treated as ssh' '
> >> >  	GIT_SSH="$TRASH_DIRECTORY/uplink" &&
> >> >  	test_when_finished "GIT_SSH=\"\$TRASH_DIRECTORY/ssh\$X\"" &&
> >> >  	git clone "[myhost:123]:src" ssh-bracket-clone-sshlike-uplink &&
> >> > -	expect_ssh "-p 123" myhost src
> >> > +	expect_ssh "-o SendEnv=GIT_PROTOCOL -p 123" myhost src
> >> >  '
> >> >
> >> >  test_expect_success 'plink is treated specially (as putty)' '
> >> > @@ -446,14 +446,14 @@ test_expect_success 'GIT_SSH_VARIANT overrides plink detection' '
> >> >  	copy_ssh_wrapper_as "$TRASH_DIRECTORY/plink" &&
> >> >  	GIT_SSH_VARIANT=ssh \
> >> >  	git clone "[myhost:123]:src" ssh-bracket-clone-variant-1 &&
> >> > -	expect_ssh "-p 123" myhost src
> >> > +	expect_ssh "-o SendEnv=GIT_PROTOCOL -p 123" myhost src
> >> >  '
> >> >
> >> >  test_expect_success 'ssh.variant overrides plink detection' '
> >> >  	copy_ssh_wrapper_as "$TRASH_DIRECTORY/plink" &&
> >> >  	git -c ssh.variant=ssh \
> >> >  		clone "[myhost:123]:src" ssh-bracket-clone-variant-2 &&
> >> > -	expect_ssh "-p 123" myhost src
> >> > +	expect_ssh "-o SendEnv=GIT_PROTOCOL -p 123" myhost src
> >> >  '
> >> >
> >> >  test_expect_success 'GIT_SSH_VARIANT overrides plink detection to plink' '
> >> > @@ -488,7 +488,7 @@ test_clone_url () {
> >> >  }
> >> >
> >> >  test_expect_success !MINGW 'clone c:temp is ssl' '
> >> > -	test_clone_url c:temp c temp
> >> > +	test_clone_url c:temp "-o SendEnv=GIT_PROTOCOL" c temp
> >> >  '
> >> >
> >> >  test_expect_success MINGW 'clone c:temp is dos drive' '
> >> > @@ -499,7 +499,7 @@ test_expect_success MINGW 'clone c:temp is dos drive' '
> >> >  for repo in rep rep/home/project 123
> >> >  do
> >> >  	test_expect_success "clone host:$repo" '
> >> > -		test_clone_url host:$repo host $repo
> >> > +		test_clone_url host:$repo "-o SendEnv=GIT_PROTOCOL" host $repo
> >> >  	'
> >> >  done
> >> >
> >> > @@ -507,16 +507,16 @@ done
> >> >  for repo in rep rep/home/project 123
> >> >  do
> >> >  	test_expect_success "clone [::1]:$repo" '
> >> > -		test_clone_url [::1]:$repo ::1 "$repo"
> >> > +		test_clone_url [::1]:$repo "-o SendEnv=GIT_PROTOCOL" ::1 "$repo"
> >> >  	'
> >> >  done
> >> >  #home directory
> >> >  test_expect_success "clone host:/~repo" '
> >> > -	test_clone_url host:/~repo host "~repo"
> >> > +	test_clone_url host:/~repo "-o SendEnv=GIT_PROTOCOL" host "~repo"
> >> >  '
> >> >
> >> >  test_expect_success "clone [::1]:/~repo" '
> >> > -	test_clone_url [::1]:/~repo ::1 "~repo"
> >> > +	test_clone_url [::1]:/~repo "-o SendEnv=GIT_PROTOCOL" ::1 "~repo"
> >> >  '
> >> >
> >> >  # Corner cases
> >> > @@ -532,22 +532,22 @@ done
> >> >  for tcol in "" :
> >> >  do
> >> >  	test_expect_success "clone ssh://host.xz$tcol/home/user/repo" '
> >> > -		test_clone_url "ssh://host.xz$tcol/home/user/repo" host.xz /home/user/repo
> >> > +		test_clone_url "ssh://host.xz$tcol/home/user/repo" "-o SendEnv=GIT_PROTOCOL" host.xz /home/user/repo
> >> >  	'
> >> >  	# from home directory
> >> >  	test_expect_success "clone ssh://host.xz$tcol/~repo" '
> >> > -	test_clone_url "ssh://host.xz$tcol/~repo" host.xz "~repo"
> >> > +	test_clone_url "ssh://host.xz$tcol/~repo" "-o SendEnv=GIT_PROTOCOL" host.xz "~repo"
> >> >  '
> >> >  done
> >> >
> >> >  # with port number
> >> >  test_expect_success 'clone ssh://host.xz:22/home/user/repo' '
> >> > -	test_clone_url "ssh://host.xz:22/home/user/repo" "-p 22 host.xz" "/home/user/repo"
> >> > +	test_clone_url "ssh://host.xz:22/home/user/repo" "-o SendEnv=GIT_PROTOCOL -p 22 host.xz" "/home/user/repo"
> >> >  '
> >> >
> >> >  # from home directory with port number
> >> >  test_expect_success 'clone ssh://host.xz:22/~repo' '
> >> > -	test_clone_url "ssh://host.xz:22/~repo" "-p 22 host.xz" "~repo"
> >> > +	test_clone_url "ssh://host.xz:22/~repo" "-o SendEnv=GIT_PROTOCOL -p 22 host.xz" "~repo"
> >> >  '
> >> >
> >> >  #IPv6
> >> > @@ -555,7 +555,7 @@ for tuah in ::1 [::1] [::1]: user@::1 user@[::1] user@[::1]: [user@::1] [user@::
> >> >  do
> >> >  	ehost=$(echo $tuah | sed -e "s/1]:/1]/" | tr -d "[]")
> >> >  	test_expect_success "clone ssh://$tuah/home/user/repo" "
> >> > -	  test_clone_url ssh://$tuah/home/user/repo $ehost /home/user/repo
> >> > +	  test_clone_url ssh://$tuah/home/user/repo '-o SendEnv=GIT_PROTOCOL' $ehost /home/user/repo
> >> >  	"
> >> >  done
> >> >
> >> > @@ -564,7 +564,7 @@ for tuah in ::1 [::1] user@::1 user@[::1] [user@::1]
> >> >  do
> >> >  	euah=$(echo $tuah | tr -d "[]")
> >> >  	test_expect_success "clone ssh://$tuah/~repo" "
> >> > -	  test_clone_url ssh://$tuah/~repo $euah '~repo'
> >> > +	  test_clone_url ssh://$tuah/~repo '-o SendEnv=GIT_PROTOCOL' $euah '~repo'
> >> >  	"
> >> >  done
> >> >
> >> > @@ -573,7 +573,7 @@ for tuah in [::1] user@[::1] [user@::1]
> >> >  do
> >> >  	euah=$(echo $tuah | tr -d "[]")
> >> >  	test_expect_success "clone ssh://$tuah:22/home/user/repo" "
> >> > -	  test_clone_url ssh://$tuah:22/home/user/repo '-p 22' $euah /home/user/repo
> >> > +	  test_clone_url ssh://$tuah:22/home/user/repo '-o SendEnv=GIT_PROTOCOL -p 22' $euah /home/user/repo
> >> >  	"
> >> >  done
> >> >
> >> > @@ -582,7 +582,7 @@ for tuah in [::1] user@[::1] [user@::1]
> >> >  do
> >> >  	euah=$(echo $tuah | tr -d "[]")
> >> >  	test_expect_success "clone ssh://$tuah:22/~repo" "
> >> > -	  test_clone_url ssh://$tuah:22/~repo '-p 22' $euah '~repo'
> >> > +	  test_clone_url ssh://$tuah:22/~repo '-o SendEnv=GIT_PROTOCOL -p 22' $euah '~repo'
> >> >  	"
> >> >  done
> >>
> >> ...so now we're unconditionally going to SendEnv=GIT_PROTOCOL to "ssh"
> >> invocations. I don't have an issue with this, but given the change in
> >> the commit message this seems to have snuck under the radar. You're just
> >> talking about always including the version in server responses, nothing
> >> about the client always needing SendEnv=GIT_PROTOCOL now even with v0.
> >
> > Ack. I'll make sure that V6 calls this out in the commit message.
> >
> >
> >> If the server always sends the version now, why don't you need to amend
> >> the same t5400-send-pack.sh tests as in my "tests: mark & fix tests
> >> broken under GIT_TEST_PROTOCOL_VERSION=1"? There's one that spews out
> >> "version" there under my GIT_TEST_PROTOCOL_VERSION=1.
> >
> > Sorry if I'm being dense, but can you point out more specifically what
> > is wrong here? I don't see anything in t5400 that would be affected by
> > this; the final test case ("receive-pack de-dupes .have lines") is the
> > only one that does any tracing, and it strips out everything that's not
> > a ref advertisement before checking the output. Sorry if I'm missing
> > something obvious.
> 
> I think I'm just misunderstanding this bit:
> 
>     Additionally, remove special cases around advertising version=0.
>     Previously we avoided adding version negotiation fields in server
>     responses if it looked like the client wanted v0. However, including
>     these fields does not change behavior, so it's better to have
>     simpler code.

Ah yes, that is a bad description, which I will fix in V6. It should
replace "server responses" to instead be "client requests". Sorry for
the confusion, that was a really silly mistake for me to make.

Basically, in the current master, clients will not add a version
advertisement to their request if their only supported version is 0.
With this patch, they will always include a version advertisement.

> I meant that if you pick the series I have up to "tests: add a special
> setup that sets protocol.version", which is at c6f33984fc in a WIP
> branch in github.com/avar/git.git and run:
> 
>     $ GIT_TEST_PROTOCOL_VERSION=1 ./t5400-send-pack.sh -v -i -x
> 
> It'll fail with:
> 
>     + test_cmp expect refs
>     + diff -u expect refs
>     --- expect      2018-12-14 22:26:52.485411992 +0000
>     +++ refs        2018-12-14 22:26:52.713412148 +0000
>     @@ -1,3 +1,4 @@
>     +version 1
>      5285e1710002a06a379056b0d21357a999f3c642 refs/heads/master
>      5285e1710002a06a379056b0d21357a999f3c642 refs/remotes/origin/HEAD
>      5285e1710002a06a379056b0d21357a999f3c642 refs/remotes/origin/master
>     error: last command exited with $?=1
>     not ok 16 - receive-pack de-dupes .have lines

I believe the reason that my series doesn't fail on this test while
yours does with GIT_TEST_PROTOCOL_VERSION=1 is because the "fork" repo
in the test is a clone from the local filesystem. So in my case the
version negotiation differences are not visible in the trace output of
"git push" (since the advert should be in an environment variable in
this case).

> And I read your commit message to mean "v0 clients also support v1
> responses with the version in them, so let's just always send it". But I
> think this is my confusion (but I still don't know what it *really*
> means).

Yeah again, a dumb mistake in the description on my part. We are only
changing whether the client sends a version advertisement. If everything
else is equal, there should be no behavior changes apart from the
version advertisement.

> >> I was worried about this breaking GIT_SSH_COMMAND, but then I see due to
> >> an interaction with picking "what ssh implementation?" we don't pass "-G
> >> -o SendEnv=GIT_PROTOCOL" at all when I have a GIT_SSH_COMMAND, but *do*
> >> pass it to my normal /usr/bin/ssh. Is this intended? Now if I have a
> >> GIT_SSH_COMMAND that expects to wrap openssh I need to pass "-c
> >> ssh.variant=ssh", because "-c ssh.variant=auto" will now omit these new
> >> arguments.
> >
> > I am not an expert on this part of the code, but it seems to be
> > intended. Later on in the function, there are BUG()s that state that
> > VARIANT_AUTO should not be passed to the push_ssh_options() function.
> > And this only changes the behavior when version=0. For protocol versions
> > 1 & 2, VARIANT_AUTO never had SendEnv=GIT_PROTOCOL added to the command
> > line. Again, sorry if I'm missing some implication here.
> 
> I'm wondering if we need to worry about some new odd interactions
> between client/servers when using GIT_SSH* wrappers, maybe not, but
> e.g. as opposed to 0da0e49ba1 ("ssh: 'auto' variant to select between
> 'ssh' and 'simple'", 2017-11-20) which noted a similar change had been
> tested at Google internally (and I read it as Google using GIT_SSH* on
> workstations) your commit doesn't make any mention of this case being
> tested / considered.
> 
> So do we need to worry about some new interaction here? Maybe not. Just
> something for people more experienced with the ssh integration to chime
> in on.

After taking a look at 0da0e49ba1 (thanks for the find) and reading
fill_ssh_args(), it seems to me that this is the intended behavior.
ssh.variant=auto means that we will run an initial "$ssh_command -G" to
see if the wrapper seems to handle OpenSSH arguments; if so we will
switch the variant to VARIANT_SSH and run again with the proper options
(including SendEnv=GIT_PROTOCOL).

[PATCH v6 0/1] Advertise multiple supported proto versions

[Josh Steadmon]

Add a registry of supported wire protocol versions that individual
commands can use to declare supported versions before contacting a
server. The client will then advertise all supported versions, while the
server will choose the first allowed version from the advertised
list.

Every command that acts as a client or server must now register its
supported protocol versions.

Changes since V4: remove special cases around advertising version=0.

Changes since V5: no code changes, but corrected and clarified commit
  message regarding changes around advertising version=0.

Josh Steadmon (1):
  protocol: advertise multiple supported versions

 builtin/archive.c           |   3 +
 builtin/clone.c             |   4 ++
 builtin/fetch-pack.c        |   4 ++
 builtin/fetch.c             |   5 ++
 builtin/ls-remote.c         |   5 ++
 builtin/pull.c              |   5 ++
 builtin/push.c              |   4 ++
 builtin/receive-pack.c      |   3 +
 builtin/send-pack.c         |   3 +
 builtin/upload-archive.c    |   3 +
 builtin/upload-pack.c       |   4 ++
 connect.c                   |  52 +++++++--------
 protocol.c                  | 122 +++++++++++++++++++++++++++++++++---
 protocol.h                  |  23 ++++++-
 remote-curl.c               |  27 +++++---
 t/t5551-http-fetch-smart.sh |   1 +
 t/t5570-git-daemon.sh       |   2 +-
 t/t5601-clone.sh            |  38 +++++------
 t/t5700-protocol-v1.sh      |   8 +--
 t/t5702-protocol-v2.sh      |  16 +++--
 transport-helper.c          |   6 ++
 21 files changed, 256 insertions(+), 82 deletions(-)

Range-diff against v5:
1:  60f6f2fbd8 ! 1:  10039ca163 protocol: advertise multiple supported versions
    @@ -22,10 +22,12 @@
         select the first allowed version from this advertisement.
     
         Additionally, remove special cases around advertising version=0.
    -    Previously we avoided adding version negotiation fields in server
    -    responses if it looked like the client wanted v0. However, including
    -    these fields does not change behavior, so it's better to have simpler
    -    code.
    +    Previously we avoided adding version advertisements to the client's
    +    initial connection request if the client wanted version=0. However,
    +    including these advertisements does not change the version negotiation
    +    behavior, so it's better to have simpler code. As a side effect, this
    +    means that client operations over SSH will always include a
    +    "SendEnv=GIT_PROTOCOL" option on the SSH command line.
     
         While we're at it, remove unnecessary externs from function declarations
         in protocol.h.
-- 
2.20.1.415.g653613c723-goog

[PATCH v6 1/1] protocol: advertise multiple supported versions

[Josh Steadmon]

Currently the client advertises that it supports the wire protocol
version set in the protocol.version config. However, not all services
support the same set of protocol versions. For example, git-receive-pack
supports v1 and v0, but not v2. If a client connects to git-receive-pack
and requests v2, it will instead be downgraded to v0. Other services,
such as git-upload-archive, do not do any version negotiation checks.

This patch creates a protocol version registry. Individual client and
server programs register all the protocol versions they support prior to
communicating with a remote instance. Versions should be listed in
preference order; the version specified in protocol.version will
automatically be moved to the front of the registry.

The protocol version registry is passed to remote helpers via the
GIT_PROTOCOL environment variable.

Clients now advertise the full list of registered versions. Servers
select the first allowed version from this advertisement.

Additionally, remove special cases around advertising version=0.
Previously we avoided adding version advertisements to the client's
initial connection request if the client wanted version=0. However,
including these advertisements does not change the version negotiation
behavior, so it's better to have simpler code. As a side effect, this
means that client operations over SSH will always include a
"SendEnv=GIT_PROTOCOL" option on the SSH command line.

While we're at it, remove unnecessary externs from function declarations
in protocol.h.

Signed-off-by: Josh Steadmon <steadmon@google.com>
---
 builtin/archive.c           |   3 +
 builtin/clone.c             |   4 ++
 builtin/fetch-pack.c        |   4 ++
 builtin/fetch.c             |   5 ++
 builtin/ls-remote.c         |   5 ++
 builtin/pull.c              |   5 ++
 builtin/push.c              |   4 ++
 builtin/receive-pack.c      |   3 +
 builtin/send-pack.c         |   3 +
 builtin/upload-archive.c    |   3 +
 builtin/upload-pack.c       |   4 ++
 connect.c                   |  52 +++++++--------
 protocol.c                  | 122 +++++++++++++++++++++++++++++++++---
 protocol.h                  |  23 ++++++-
 remote-curl.c               |  27 +++++---
 t/t5551-http-fetch-smart.sh |   1 +
 t/t5570-git-daemon.sh       |   2 +-
 t/t5601-clone.sh            |  38 +++++------
 t/t5700-protocol-v1.sh      |   8 +--
 t/t5702-protocol-v2.sh      |  16 +++--
 transport-helper.c          |   6 ++
 21 files changed, 256 insertions(+), 82 deletions(-)

diff --git a/builtin/archive.c b/builtin/archive.c
index e74f675390..8adb9f381b 100644
--- a/builtin/archive.c
+++ b/builtin/archive.c
@@ -8,6 +8,7 @@
 #include "transport.h"
 #include "parse-options.h"
 #include "pkt-line.h"
+#include "protocol.h"
 #include "sideband.h"
 
 static void create_output_file(const char *output_file)
@@ -94,6 +95,8 @@ int cmd_archive(int argc, const char **argv, const char *prefix)
 		OPT_END()
 	};
 
+	register_allowed_protocol_version(protocol_v0);
+
 	argc = parse_options(argc, argv, prefix, local_opts, NULL,
 			     PARSE_OPT_KEEP_ALL);
 
diff --git a/builtin/clone.c b/builtin/clone.c
index fd2c3ef090..1651a950b6 100644
--- a/builtin/clone.c
+++ b/builtin/clone.c
@@ -900,6 +900,10 @@ int cmd_clone(int argc, const char **argv, const char *prefix)
 	struct refspec rs = REFSPEC_INIT_FETCH;
 	struct argv_array ref_prefixes = ARGV_ARRAY_INIT;
 
+	register_allowed_protocol_version(protocol_v2);
+	register_allowed_protocol_version(protocol_v1);
+	register_allowed_protocol_version(protocol_v0);
+
 	fetch_if_missing = 0;
 
 	packet_trace_identity("clone");
diff --git a/builtin/fetch-pack.c b/builtin/fetch-pack.c
index 1a1bc63566..cba935e4d3 100644
--- a/builtin/fetch-pack.c
+++ b/builtin/fetch-pack.c
@@ -57,6 +57,10 @@ int cmd_fetch_pack(int argc, const char **argv, const char *prefix)
 
 	fetch_if_missing = 0;
 
+	register_allowed_protocol_version(protocol_v2);
+	register_allowed_protocol_version(protocol_v1);
+	register_allowed_protocol_version(protocol_v0);
+
 	packet_trace_identity("fetch-pack");
 
 	memset(&args, 0, sizeof(args));
diff --git a/builtin/fetch.c b/builtin/fetch.c
index 61bec5d213..2a20cf8bfd 100644
--- a/builtin/fetch.c
+++ b/builtin/fetch.c
@@ -21,6 +21,7 @@
 #include "argv-array.h"
 #include "utf8.h"
 #include "packfile.h"
+#include "protocol.h"
 #include "list-objects-filter-options.h"
 
 static const char * const builtin_fetch_usage[] = {
@@ -1476,6 +1477,10 @@ int cmd_fetch(int argc, const char **argv, const char *prefix)
 	int prune_tags_ok = 1;
 	struct argv_array argv_gc_auto = ARGV_ARRAY_INIT;
 
+	register_allowed_protocol_version(protocol_v2);
+	register_allowed_protocol_version(protocol_v1);
+	register_allowed_protocol_version(protocol_v0);
+
 	packet_trace_identity("fetch");
 
 	fetch_if_missing = 0;
diff --git a/builtin/ls-remote.c b/builtin/ls-remote.c
index 1a25df7ee1..ea685e8bb9 100644
--- a/builtin/ls-remote.c
+++ b/builtin/ls-remote.c
@@ -1,5 +1,6 @@
 #include "builtin.h"
 #include "cache.h"
+#include "protocol.h"
 #include "transport.h"
 #include "ref-filter.h"
 #include "remote.h"
@@ -80,6 +81,10 @@ int cmd_ls_remote(int argc, const char **argv, const char *prefix)
 
 	memset(&ref_array, 0, sizeof(ref_array));
 
+	register_allowed_protocol_version(protocol_v2);
+	register_allowed_protocol_version(protocol_v1);
+	register_allowed_protocol_version(protocol_v0);
+
 	argc = parse_options(argc, argv, prefix, options, ls_remote_usage,
 			     PARSE_OPT_STOP_AT_NON_OPTION);
 	dest = argv[0];
diff --git a/builtin/pull.c b/builtin/pull.c
index 681c127a07..cb64146834 100644
--- a/builtin/pull.c
+++ b/builtin/pull.c
@@ -9,6 +9,7 @@
 #include "config.h"
 #include "builtin.h"
 #include "parse-options.h"
+#include "protocol.h"
 #include "exec-cmd.h"
 #include "run-command.h"
 #include "sha1-array.h"
@@ -849,6 +850,10 @@ int cmd_pull(int argc, const char **argv, const char *prefix)
 	struct object_id rebase_fork_point;
 	int autostash;
 
+	register_allowed_protocol_version(protocol_v2);
+	register_allowed_protocol_version(protocol_v1);
+	register_allowed_protocol_version(protocol_v0);
+
 	if (!getenv("GIT_REFLOG_ACTION"))
 		set_reflog_message(argc, argv);
 
diff --git a/builtin/push.c b/builtin/push.c
index d09a42062c..10d8abe829 100644
--- a/builtin/push.c
+++ b/builtin/push.c
@@ -10,6 +10,7 @@
 #include "remote.h"
 #include "transport.h"
 #include "parse-options.h"
+#include "protocol.h"
 #include "submodule.h"
 #include "submodule-config.h"
 #include "send-pack.h"
@@ -587,6 +588,9 @@ int cmd_push(int argc, const char **argv, const char *prefix)
 		OPT_END()
 	};
 
+	register_allowed_protocol_version(protocol_v1);
+	register_allowed_protocol_version(protocol_v0);
+
 	packet_trace_identity("push");
 	git_config(git_push_config, &flags);
 	argc = parse_options(argc, argv, prefix, options, push_usage, 0);
diff --git a/builtin/receive-pack.c b/builtin/receive-pack.c
index c17ce94e12..030cb7b7ec 100644
--- a/builtin/receive-pack.c
+++ b/builtin/receive-pack.c
@@ -1929,6 +1929,9 @@ int cmd_receive_pack(int argc, const char **argv, const char *prefix)
 
 	packet_trace_identity("receive-pack");
 
+	register_allowed_protocol_version(protocol_v1);
+	register_allowed_protocol_version(protocol_v0);
+
 	argc = parse_options(argc, argv, prefix, options, receive_pack_usage, 0);
 
 	if (argc > 1)
diff --git a/builtin/send-pack.c b/builtin/send-pack.c
index 8e3c7490f7..f48bd1306b 100644
--- a/builtin/send-pack.c
+++ b/builtin/send-pack.c
@@ -184,6 +184,9 @@ int cmd_send_pack(int argc, const char **argv, const char *prefix)
 		OPT_END()
 	};
 
+	register_allowed_protocol_version(protocol_v1);
+	register_allowed_protocol_version(protocol_v0);
+
 	git_config(send_pack_config, NULL);
 	argc = parse_options(argc, argv, prefix, options, send_pack_usage, 0);
 	if (argc > 0) {
diff --git a/builtin/upload-archive.c b/builtin/upload-archive.c
index 25d9116356..791cbe80a6 100644
--- a/builtin/upload-archive.c
+++ b/builtin/upload-archive.c
@@ -5,6 +5,7 @@
 #include "builtin.h"
 #include "archive.h"
 #include "pkt-line.h"
+#include "protocol.h"
 #include "sideband.h"
 #include "run-command.h"
 #include "argv-array.h"
@@ -80,6 +81,8 @@ int cmd_upload_archive(int argc, const char **argv, const char *prefix)
 	if (argc == 2 && !strcmp(argv[1], "-h"))
 		usage(upload_archive_usage);
 
+	register_allowed_protocol_version(protocol_v0);
+
 	/*
 	 * Set up sideband subprocess.
 	 *
diff --git a/builtin/upload-pack.c b/builtin/upload-pack.c
index 42dc4da5a1..293dd45b9e 100644
--- a/builtin/upload-pack.c
+++ b/builtin/upload-pack.c
@@ -33,6 +33,10 @@ int cmd_upload_pack(int argc, const char **argv, const char *prefix)
 	packet_trace_identity("upload-pack");
 	read_replace_refs = 0;
 
+	register_allowed_protocol_version(protocol_v2);
+	register_allowed_protocol_version(protocol_v1);
+	register_allowed_protocol_version(protocol_v0);
+
 	argc = parse_options(argc, argv, NULL, options, upload_pack_usage, 0);
 
 	if (argc != 1)
diff --git a/connect.c b/connect.c
index 94547e5056..57266b6cec 100644
--- a/connect.c
+++ b/connect.c
@@ -1046,7 +1046,7 @@ static enum ssh_variant determine_ssh_variant(const char *ssh_command,
  */
 static struct child_process *git_connect_git(int fd[2], char *hostandport,
 					     const char *path, const char *prog,
-					     enum protocol_version version,
+					     const struct strbuf *version_advert,
 					     int flags)
 {
 	struct child_process *conn;
@@ -1084,12 +1084,9 @@ static struct child_process *git_connect_git(int fd[2], char *hostandport,
 		    prog, path, 0,
 		    target_host, 0);
 
-	/* If using a new version put that stuff here after a second null byte */
-	if (version > 0) {
-		strbuf_addch(&request, '\0');
-		strbuf_addf(&request, "version=%d%c",
-			    version, '\0');
-	}
+	/* Add version fields after a second null byte */
+	strbuf_addch(&request, '\0');
+	strbuf_addf(&request, "%s%c", version_advert->buf, '\0');
 
 	packet_write(fd[1], request.buf, request.len);
 
@@ -1104,14 +1101,13 @@ static struct child_process *git_connect_git(int fd[2], char *hostandport,
  */
 static void push_ssh_options(struct argv_array *args, struct argv_array *env,
 			     enum ssh_variant variant, const char *port,
-			     enum protocol_version version, int flags)
+			     const struct strbuf *version_advert, int flags)
 {
-	if (variant == VARIANT_SSH &&
-	    version > 0) {
+	if (variant == VARIANT_SSH) {
 		argv_array_push(args, "-o");
 		argv_array_push(args, "SendEnv=" GIT_PROTOCOL_ENVIRONMENT);
-		argv_array_pushf(env, GIT_PROTOCOL_ENVIRONMENT "=version=%d",
-				 version);
+		argv_array_pushf(env, GIT_PROTOCOL_ENVIRONMENT "=%s",
+				 version_advert->buf);
 	}
 
 	if (flags & CONNECT_IPV4) {
@@ -1164,7 +1160,7 @@ static void push_ssh_options(struct argv_array *args, struct argv_array *env,
 
 /* Prepare a child_process for use by Git's SSH-tunneled transport. */
 static void fill_ssh_args(struct child_process *conn, const char *ssh_host,
-			  const char *port, enum protocol_version version,
+			  const char *port, const struct strbuf *version_advert,
 			  int flags)
 {
 	const char *ssh;
@@ -1198,15 +1194,16 @@ static void fill_ssh_args(struct child_process *conn, const char *ssh_host,
 
 		argv_array_push(&detect.args, ssh);
 		argv_array_push(&detect.args, "-G");
-		push_ssh_options(&detect.args, &detect.env_array,
-				 VARIANT_SSH, port, version, flags);
+		push_ssh_options(&detect.args, &detect.env_array, VARIANT_SSH,
+				 port, version_advert, flags);
 		argv_array_push(&detect.args, ssh_host);
 
 		variant = run_command(&detect) ? VARIANT_SIMPLE : VARIANT_SSH;
 	}
 
 	argv_array_push(&conn->args, ssh);
-	push_ssh_options(&conn->args, &conn->env_array, variant, port, version, flags);
+	push_ssh_options(&conn->args, &conn->env_array, variant, port,
+			 version_advert, flags);
 	argv_array_push(&conn->args, ssh_host);
 }
 
@@ -1226,16 +1223,10 @@ struct child_process *git_connect(int fd[2], const char *url,
 {
 	char *hostandport, *path;
 	struct child_process *conn;
+	struct strbuf version_advert = STRBUF_INIT;
 	enum protocol protocol;
-	enum protocol_version version = get_protocol_version_config();
 
-	/*
-	 * NEEDSWORK: If we are trying to use protocol v2 and we are planning
-	 * to perform a push, then fallback to v0 since the client doesn't know
-	 * how to push yet using v2.
-	 */
-	if (version == protocol_v2 && !strcmp("git-receive-pack", prog))
-		version = protocol_v0;
+	get_client_protocol_version_advertisement(&version_advert);
 
 	/* Without this we cannot rely on waitpid() to tell
 	 * what happened to our children.
@@ -1250,7 +1241,8 @@ struct child_process *git_connect(int fd[2], const char *url,
 		printf("Diag: path=%s\n", path ? path : "NULL");
 		conn = NULL;
 	} else if (protocol == PROTO_GIT) {
-		conn = git_connect_git(fd, hostandport, path, prog, version, flags);
+		conn = git_connect_git(fd, hostandport, path, prog,
+				       &version_advert, flags);
 	} else {
 		struct strbuf cmd = STRBUF_INIT;
 		const char *const *var;
@@ -1293,13 +1285,13 @@ struct child_process *git_connect(int fd[2], const char *url,
 				strbuf_release(&cmd);
 				return NULL;
 			}
-			fill_ssh_args(conn, ssh_host, port, version, flags);
+			fill_ssh_args(conn, ssh_host, port, &version_advert,
+				      flags);
 		} else {
 			transport_check_allowed("file");
-			if (version > 0) {
-				argv_array_pushf(&conn->env_array, GIT_PROTOCOL_ENVIRONMENT "=version=%d",
-						 version);
-			}
+			argv_array_pushf(&conn->env_array,
+					 GIT_PROTOCOL_ENVIRONMENT "=%s",
+					 version_advert.buf);
 		}
 		argv_array_push(&conn->args, cmd.buf);
 
diff --git a/protocol.c b/protocol.c
index 5e636785d1..5664bd7a05 100644
--- a/protocol.c
+++ b/protocol.c
@@ -2,18 +2,43 @@
 #include "config.h"
 #include "protocol.h"
 
+static enum protocol_version *allowed_versions;
+static int nr_allowed_versions;
+static int alloc_allowed_versions;
+static int version_registration_locked = 0;
+
+static const char protocol_v0_string[] = "0";
+static const char protocol_v1_string[] = "1";
+static const char protocol_v2_string[] = "2";
+
 static enum protocol_version parse_protocol_version(const char *value)
 {
-	if (!strcmp(value, "0"))
+	if (!strcmp(value, protocol_v0_string))
 		return protocol_v0;
-	else if (!strcmp(value, "1"))
+	else if (!strcmp(value, protocol_v1_string))
 		return protocol_v1;
-	else if (!strcmp(value, "2"))
+	else if (!strcmp(value, protocol_v2_string))
 		return protocol_v2;
 	else
 		return protocol_unknown_version;
 }
 
+/* Return the text representation of a wire protocol version. */
+static const char *format_protocol_version(enum protocol_version version)
+{
+	switch (version) {
+	case protocol_v0:
+		return protocol_v0_string;
+	case protocol_v1:
+		return protocol_v1_string;
+	case protocol_v2:
+		return protocol_v2_string;
+	case protocol_unknown_version:
+		die(_("Unrecognized protocol version"));
+	}
+	die(_("Unrecognized protocol_version"));
+}
+
 enum protocol_version get_protocol_version_config(void)
 {
 	const char *value;
@@ -30,6 +55,85 @@ enum protocol_version get_protocol_version_config(void)
 	return protocol_v0;
 }
 
+void register_allowed_protocol_version(enum protocol_version version)
+{
+	if (version_registration_locked)
+		BUG("late attempt to register an allowed protocol version");
+
+	ALLOC_GROW(allowed_versions, nr_allowed_versions + 1,
+		   alloc_allowed_versions);
+	allowed_versions[nr_allowed_versions++] = version;
+}
+
+void register_allowed_protocol_versions_from_env(void)
+{
+	const char *git_protocol = getenv(GIT_PROTOCOL_ENVIRONMENT);
+	const char *version_str;
+	struct string_list version_list = STRING_LIST_INIT_DUP;
+	struct string_list_item *version;
+
+	if (!git_protocol)
+		return;
+
+	string_list_split(&version_list, git_protocol, ':', -1);
+	for_each_string_list_item(version, &version_list) {
+		if (skip_prefix(version->string, "version=", &version_str))
+			register_allowed_protocol_version(
+				parse_protocol_version(version_str));
+	}
+	string_list_clear(&version_list, 0);
+}
+
+static int is_allowed_protocol_version(enum protocol_version version)
+{
+	int i;
+	version_registration_locked = 1;
+	for (i = 0; i < nr_allowed_versions; i++)
+		if (version == allowed_versions[i])
+			return 1;
+	return 0;
+}
+
+void get_client_protocol_version_advertisement(struct strbuf *advert)
+{
+	int i, tmp_nr = nr_allowed_versions;
+	enum protocol_version *tmp_allowed_versions, config_version;
+	strbuf_reset(advert);
+
+	version_registration_locked = 1;
+
+	config_version = get_protocol_version_config();
+	if (config_version == protocol_v0) {
+		strbuf_addstr(advert, "version=0");
+		return;
+	}
+
+	if (tmp_nr > 0) {
+		ALLOC_ARRAY(tmp_allowed_versions, tmp_nr);
+		copy_array(tmp_allowed_versions, allowed_versions, tmp_nr,
+			   sizeof(enum protocol_version));
+	} else {
+		ALLOC_ARRAY(tmp_allowed_versions, 1);
+		tmp_nr = 1;
+		tmp_allowed_versions[0] = config_version;
+	}
+
+	if (tmp_allowed_versions[0] != config_version)
+		for (i = 1; i < nr_allowed_versions; i++)
+			if (tmp_allowed_versions[i] == config_version) {
+				SWAP(tmp_allowed_versions[0],
+				     tmp_allowed_versions[i]);
+			}
+
+	strbuf_addf(advert, "version=%s",
+		    format_protocol_version(tmp_allowed_versions[0]));
+	for (i = 1; i < tmp_nr; i++)
+		strbuf_addf(advert, ":version=%s",
+			    format_protocol_version(tmp_allowed_versions[i]));
+
+	free(tmp_allowed_versions);
+}
+
 enum protocol_version determine_protocol_version_server(void)
 {
 	const char *git_protocol = getenv(GIT_PROTOCOL_ENVIRONMENT);
@@ -38,9 +142,10 @@ enum protocol_version determine_protocol_version_server(void)
 	/*
 	 * Determine which protocol version the client has requested.  Since
 	 * multiple 'version' keys can be sent by the client, indicating that
-	 * the client is okay to speak any of them, select the greatest version
-	 * that the client has requested.  This is due to the assumption that
-	 * the most recent protocol version will be the most state-of-the-art.
+	 * the client is okay to speak any of them, select the first
+	 * recognizable version that the client has requested.  This is due to
+	 * the assumption that the protocol versions will be listed in
+	 * preference order.
 	 */
 	if (git_protocol) {
 		struct string_list list = STRING_LIST_INIT_DUP;
@@ -53,8 +158,11 @@ enum protocol_version determine_protocol_version_server(void)
 
 			if (skip_prefix(item->string, "version=", &value)) {
 				v = parse_protocol_version(value);
-				if (v > version)
+				if (v != protocol_unknown_version &&
+				    is_allowed_protocol_version(v)) {
 					version = v;
+					break;
+				}
 			}
 		}
 
diff --git a/protocol.h b/protocol.h
index 2ad35e433c..88330fd0ee 100644
--- a/protocol.h
+++ b/protocol.h
@@ -14,7 +14,24 @@ enum protocol_version {
  * 'protocol.version' config.  If unconfigured, a value of 'protocol_v0' is
  * returned.
  */
-extern enum protocol_version get_protocol_version_config(void);
+enum protocol_version get_protocol_version_config(void);
+
+/*
+ * Register an allowable protocol version for a given operation. Registration
+ * must occur before attempting to advertise a version to a server process.
+ */
+void register_allowed_protocol_version(enum protocol_version version);
+
+/*
+ * Register allowable protocol versions from the GIT_PROTOCOL environment var.
+ */
+void register_allowed_protocol_versions_from_env(void);
+
+/*
+ * Fill a strbuf with a version advertisement string suitable for use in the
+ * GIT_PROTOCOL environment variable or similar version negotiation field.
+ */
+void get_client_protocol_version_advertisement(struct strbuf *advert);
 
 /*
  * Used by a server to determine which protocol version should be used based on
@@ -23,12 +40,12 @@ extern enum protocol_version get_protocol_version_config(void);
  * request a particular protocol version, a default of 'protocol_v0' will be
  * used.
  */
-extern enum protocol_version determine_protocol_version_server(void);
+enum protocol_version determine_protocol_version_server(void);
 
 /*
  * Used by a client to determine which protocol version the server is speaking
  * based on the server's initial response.
  */
-extern enum protocol_version determine_protocol_version_client(const char *server_response);
+enum protocol_version determine_protocol_version_client(const char *server_response);
 
 #endif /* PROTOCOL_H */
diff --git a/remote-curl.c b/remote-curl.c
index fb28309e85..6ffefe5169 100644
--- a/remote-curl.c
+++ b/remote-curl.c
@@ -330,6 +330,19 @@ static int get_protocol_http_header(enum protocol_version version,
 	return 0;
 }
 
+static int get_client_protocol_http_header(const struct strbuf *version_advert,
+					   struct strbuf *header)
+{
+	if (version_advert->len > 0) {
+		strbuf_addf(header, GIT_PROTOCOL_HEADER ": %s",
+			    version_advert->buf);
+
+		return 1;
+	}
+
+	return 0;
+}
+
 static struct discovery *discover_refs(const char *service, int for_push)
 {
 	struct strbuf exp = STRBUF_INIT;
@@ -339,11 +352,11 @@ static struct discovery *discover_refs(const char *service, int for_push)
 	struct strbuf refs_url = STRBUF_INIT;
 	struct strbuf effective_url = STRBUF_INIT;
 	struct strbuf protocol_header = STRBUF_INIT;
+	struct strbuf version_advert = STRBUF_INIT;
 	struct string_list extra_headers = STRING_LIST_INIT_DUP;
 	struct discovery *last = last_discovery;
 	int http_ret, maybe_smart = 0;
 	struct http_get_options http_options;
-	enum protocol_version version = get_protocol_version_config();
 
 	if (last && !strcmp(service, last->service))
 		return last;
@@ -360,16 +373,10 @@ static struct discovery *discover_refs(const char *service, int for_push)
 		strbuf_addf(&refs_url, "service=%s", service);
 	}
 
-	/*
-	 * NEEDSWORK: If we are trying to use protocol v2 and we are planning
-	 * to perform a push, then fallback to v0 since the client doesn't know
-	 * how to push yet using v2.
-	 */
-	if (version == protocol_v2 && !strcmp("git-receive-pack", service))
-		version = protocol_v0;
+	get_client_protocol_version_advertisement(&version_advert);
 
 	/* Add the extra Git-Protocol header */
-	if (get_protocol_http_header(version, &protocol_header))
+	if (get_client_protocol_http_header(&version_advert, &protocol_header))
 		string_list_append(&extra_headers, protocol_header.buf);
 
 	memset(&http_options, 0, sizeof(http_options));
@@ -1327,6 +1334,8 @@ int cmd_main(int argc, const char **argv)
 	struct strbuf buf = STRBUF_INIT;
 	int nongit;
 
+	register_allowed_protocol_versions_from_env();
+
 	setup_git_directory_gently(&nongit);
 	if (argc < 2) {
 		error("remote-curl: usage: git remote-curl <remote> [<url>]");
diff --git a/t/t5551-http-fetch-smart.sh b/t/t5551-http-fetch-smart.sh
index 771f36f9ff..343bf3aafa 100755
--- a/t/t5551-http-fetch-smart.sh
+++ b/t/t5551-http-fetch-smart.sh
@@ -28,6 +28,7 @@ cat >exp <<EOF
 > Accept: */*
 > Accept-Encoding: ENCODINGS
 > Pragma: no-cache
+> Git-Protocol: version=0
 < HTTP/1.1 200 OK
 < Pragma: no-cache
 < Cache-Control: no-cache, max-age=0, must-revalidate
diff --git a/t/t5570-git-daemon.sh b/t/t5570-git-daemon.sh
index 7466aad111..d528e91630 100755
--- a/t/t5570-git-daemon.sh
+++ b/t/t5570-git-daemon.sh
@@ -186,7 +186,7 @@ test_expect_success 'hostname cannot break out of directory' '
 test_expect_success 'daemon log records all attributes' '
 	cat >expect <<-\EOF &&
 	Extended attribute "host": localhost
-	Extended attribute "protocol": version=1
+	Extended attribute "protocol": version=1:version=2:version=0
 	EOF
 	>daemon.log &&
 	GIT_OVERRIDE_VIRTUAL_HOST=localhost \
diff --git a/t/t5601-clone.sh b/t/t5601-clone.sh
index ddaa96ac4f..c4dbf1f779 100755
--- a/t/t5601-clone.sh
+++ b/t/t5601-clone.sh
@@ -346,7 +346,7 @@ expect_ssh () {
 
 test_expect_success 'clone myhost:src uses ssh' '
 	git clone myhost:src ssh-clone &&
-	expect_ssh myhost src
+	expect_ssh "-o SendEnv=GIT_PROTOCOL" myhost src
 '
 
 test_expect_success !MINGW,!CYGWIN 'clone local path foo:bar' '
@@ -357,12 +357,12 @@ test_expect_success !MINGW,!CYGWIN 'clone local path foo:bar' '
 
 test_expect_success 'bracketed hostnames are still ssh' '
 	git clone "[myhost:123]:src" ssh-bracket-clone &&
-	expect_ssh "-p 123" myhost src
+	expect_ssh "-o SendEnv=GIT_PROTOCOL -p 123" myhost src
 '
 
 test_expect_success 'OpenSSH variant passes -4' '
 	git clone -4 "[myhost:123]:src" ssh-ipv4-clone &&
-	expect_ssh "-4 -p 123" myhost src
+	expect_ssh "-o SendEnv=GIT_PROTOCOL -4 -p 123" myhost src
 '
 
 test_expect_success 'variant can be overridden' '
@@ -406,7 +406,7 @@ test_expect_success 'OpenSSH-like uplink is treated as ssh' '
 	GIT_SSH="$TRASH_DIRECTORY/uplink" &&
 	test_when_finished "GIT_SSH=\"\$TRASH_DIRECTORY/ssh\$X\"" &&
 	git clone "[myhost:123]:src" ssh-bracket-clone-sshlike-uplink &&
-	expect_ssh "-p 123" myhost src
+	expect_ssh "-o SendEnv=GIT_PROTOCOL -p 123" myhost src
 '
 
 test_expect_success 'plink is treated specially (as putty)' '
@@ -446,14 +446,14 @@ test_expect_success 'GIT_SSH_VARIANT overrides plink detection' '
 	copy_ssh_wrapper_as "$TRASH_DIRECTORY/plink" &&
 	GIT_SSH_VARIANT=ssh \
 	git clone "[myhost:123]:src" ssh-bracket-clone-variant-1 &&
-	expect_ssh "-p 123" myhost src
+	expect_ssh "-o SendEnv=GIT_PROTOCOL -p 123" myhost src
 '
 
 test_expect_success 'ssh.variant overrides plink detection' '
 	copy_ssh_wrapper_as "$TRASH_DIRECTORY/plink" &&
 	git -c ssh.variant=ssh \
 		clone "[myhost:123]:src" ssh-bracket-clone-variant-2 &&
-	expect_ssh "-p 123" myhost src
+	expect_ssh "-o SendEnv=GIT_PROTOCOL -p 123" myhost src
 '
 
 test_expect_success 'GIT_SSH_VARIANT overrides plink detection to plink' '
@@ -488,7 +488,7 @@ test_clone_url () {
 }
 
 test_expect_success !MINGW 'clone c:temp is ssl' '
-	test_clone_url c:temp c temp
+	test_clone_url c:temp "-o SendEnv=GIT_PROTOCOL" c temp
 '
 
 test_expect_success MINGW 'clone c:temp is dos drive' '
@@ -499,7 +499,7 @@ test_expect_success MINGW 'clone c:temp is dos drive' '
 for repo in rep rep/home/project 123
 do
 	test_expect_success "clone host:$repo" '
-		test_clone_url host:$repo host $repo
+		test_clone_url host:$repo "-o SendEnv=GIT_PROTOCOL" host $repo
 	'
 done
 
@@ -507,16 +507,16 @@ done
 for repo in rep rep/home/project 123
 do
 	test_expect_success "clone [::1]:$repo" '
-		test_clone_url [::1]:$repo ::1 "$repo"
+		test_clone_url [::1]:$repo "-o SendEnv=GIT_PROTOCOL" ::1 "$repo"
 	'
 done
 #home directory
 test_expect_success "clone host:/~repo" '
-	test_clone_url host:/~repo host "~repo"
+	test_clone_url host:/~repo "-o SendEnv=GIT_PROTOCOL" host "~repo"
 '
 
 test_expect_success "clone [::1]:/~repo" '
-	test_clone_url [::1]:/~repo ::1 "~repo"
+	test_clone_url [::1]:/~repo "-o SendEnv=GIT_PROTOCOL" ::1 "~repo"
 '
 
 # Corner cases
@@ -532,22 +532,22 @@ done
 for tcol in "" :
 do
 	test_expect_success "clone ssh://host.xz$tcol/home/user/repo" '
-		test_clone_url "ssh://host.xz$tcol/home/user/repo" host.xz /home/user/repo
+		test_clone_url "ssh://host.xz$tcol/home/user/repo" "-o SendEnv=GIT_PROTOCOL" host.xz /home/user/repo
 	'
 	# from home directory
 	test_expect_success "clone ssh://host.xz$tcol/~repo" '
-	test_clone_url "ssh://host.xz$tcol/~repo" host.xz "~repo"
+	test_clone_url "ssh://host.xz$tcol/~repo" "-o SendEnv=GIT_PROTOCOL" host.xz "~repo"
 '
 done
 
 # with port number
 test_expect_success 'clone ssh://host.xz:22/home/user/repo' '
-	test_clone_url "ssh://host.xz:22/home/user/repo" "-p 22 host.xz" "/home/user/repo"
+	test_clone_url "ssh://host.xz:22/home/user/repo" "-o SendEnv=GIT_PROTOCOL -p 22 host.xz" "/home/user/repo"
 '
 
 # from home directory with port number
 test_expect_success 'clone ssh://host.xz:22/~repo' '
-	test_clone_url "ssh://host.xz:22/~repo" "-p 22 host.xz" "~repo"
+	test_clone_url "ssh://host.xz:22/~repo" "-o SendEnv=GIT_PROTOCOL -p 22 host.xz" "~repo"
 '
 
 #IPv6
@@ -555,7 +555,7 @@ for tuah in ::1 [::1] [::1]: user@::1 user@[::1] user@[::1]: [user@::1] [user@::
 do
 	ehost=$(echo $tuah | sed -e "s/1]:/1]/" | tr -d "[]")
 	test_expect_success "clone ssh://$tuah/home/user/repo" "
-	  test_clone_url ssh://$tuah/home/user/repo $ehost /home/user/repo
+	  test_clone_url ssh://$tuah/home/user/repo '-o SendEnv=GIT_PROTOCOL' $ehost /home/user/repo
 	"
 done
 
@@ -564,7 +564,7 @@ for tuah in ::1 [::1] user@::1 user@[::1] [user@::1]
 do
 	euah=$(echo $tuah | tr -d "[]")
 	test_expect_success "clone ssh://$tuah/~repo" "
-	  test_clone_url ssh://$tuah/~repo $euah '~repo'
+	  test_clone_url ssh://$tuah/~repo '-o SendEnv=GIT_PROTOCOL' $euah '~repo'
 	"
 done
 
@@ -573,7 +573,7 @@ for tuah in [::1] user@[::1] [user@::1]
 do
 	euah=$(echo $tuah | tr -d "[]")
 	test_expect_success "clone ssh://$tuah:22/home/user/repo" "
-	  test_clone_url ssh://$tuah:22/home/user/repo '-p 22' $euah /home/user/repo
+	  test_clone_url ssh://$tuah:22/home/user/repo '-o SendEnv=GIT_PROTOCOL -p 22' $euah /home/user/repo
 	"
 done
 
@@ -582,7 +582,7 @@ for tuah in [::1] user@[::1] [user@::1]
 do
 	euah=$(echo $tuah | tr -d "[]")
 	test_expect_success "clone ssh://$tuah:22/~repo" "
-	  test_clone_url ssh://$tuah:22/~repo '-p 22' $euah '~repo'
+	  test_clone_url ssh://$tuah:22/~repo '-o SendEnv=GIT_PROTOCOL -p 22' $euah '~repo'
 	"
 done
 
diff --git a/t/t5700-protocol-v1.sh b/t/t5700-protocol-v1.sh
index ba86a44eb1..2e56c79233 100755
--- a/t/t5700-protocol-v1.sh
+++ b/t/t5700-protocol-v1.sh
@@ -26,7 +26,7 @@ test_expect_success 'clone with git:// using protocol v1' '
 	test_cmp expect actual &&
 
 	# Client requested to use protocol v1
-	grep "clone> .*\\\0\\\0version=1\\\0$" log &&
+	grep "clone> .*\\\0\\\0version=1.*\\\0$" log &&
 	# Server responded using protocol v1
 	grep "clone< version 1" log
 '
@@ -42,7 +42,7 @@ test_expect_success 'fetch with git:// using protocol v1' '
 	test_cmp expect actual &&
 
 	# Client requested to use protocol v1
-	grep "fetch> .*\\\0\\\0version=1\\\0$" log &&
+	grep "fetch> .*\\\0\\\0version=1.*\\\0$" log &&
 	# Server responded using protocol v1
 	grep "fetch< version 1" log
 '
@@ -56,7 +56,7 @@ test_expect_success 'pull with git:// using protocol v1' '
 	test_cmp expect actual &&
 
 	# Client requested to use protocol v1
-	grep "fetch> .*\\\0\\\0version=1\\\0$" log &&
+	grep "fetch> .*\\\0\\\0version=1.*\\\0$" log &&
 	# Server responded using protocol v1
 	grep "fetch< version 1" log
 '
@@ -74,7 +74,7 @@ test_expect_success 'push with git:// using protocol v1' '
 	test_cmp expect actual &&
 
 	# Client requested to use protocol v1
-	grep "push> .*\\\0\\\0version=1\\\0$" log &&
+	grep "push> .*\\\0\\\0version=1.*\\\0$" log &&
 	# Server responded using protocol v1
 	grep "push< version 1" log
 '
diff --git a/t/t5702-protocol-v2.sh b/t/t5702-protocol-v2.sh
index 3beeed4546..78c17c25e4 100755
--- a/t/t5702-protocol-v2.sh
+++ b/t/t5702-protocol-v2.sh
@@ -24,7 +24,7 @@ test_expect_success 'list refs with git:// using protocol v2' '
 		ls-remote --symref "$GIT_DAEMON_URL/parent" >actual &&
 
 	# Client requested to use protocol v2
-	grep "git> .*\\\0\\\0version=2\\\0$" log &&
+	grep "git> .*\\\0\\\0version=2.*\\\0$" log &&
 	# Server responded using protocol v2
 	grep "git< version 2" log &&
 
@@ -56,7 +56,7 @@ test_expect_success 'clone with git:// using protocol v2' '
 	test_cmp expect actual &&
 
 	# Client requested to use protocol v2
-	grep "clone> .*\\\0\\\0version=2\\\0$" log &&
+	grep "clone> .*\\\0\\\0version=2.*\\\0$" log &&
 	# Server responded using protocol v2
 	grep "clone< version 2" log
 '
@@ -74,7 +74,7 @@ test_expect_success 'fetch with git:// using protocol v2' '
 	test_cmp expect actual &&
 
 	# Client requested to use protocol v2
-	grep "fetch> .*\\\0\\\0version=2\\\0$" log &&
+	grep "fetch> .*\\\0\\\0version=2.*\\\0$" log &&
 	# Server responded using protocol v2
 	grep "fetch< version 2" log
 '
@@ -90,7 +90,7 @@ test_expect_success 'pull with git:// using protocol v2' '
 	test_cmp expect actual &&
 
 	# Client requested to use protocol v2
-	grep "fetch> .*\\\0\\\0version=2\\\0$" log &&
+	grep "fetch> .*\\\0\\\0version=2.*\\\0$" log &&
 	# Server responded using protocol v2
 	grep "fetch< version 2" log
 '
@@ -476,7 +476,7 @@ test_expect_success 'push with http:// and a config of v2 does not request v2' '
 	test_when_finished "rm -f log" &&
 	# Till v2 for push is designed, make sure that if a client has
 	# protocol.version configured to use v2, that the client instead falls
-	# back and uses v0.
+	# back to previous versions.
 
 	test_commit -C http_child three &&
 
@@ -489,10 +489,8 @@ test_expect_success 'push with http:// and a config of v2 does not request v2' '
 	git -C "$HTTPD_DOCUMENT_ROOT_PATH/http_parent" log -1 --format=%s client_branch >expect &&
 	test_cmp expect actual &&
 
-	# Client didnt request to use protocol v2
-	! grep "Git-Protocol: version=2" log &&
-	# Server didnt respond using protocol v2
-	! grep "git< version 2" log
+	# Server responded with version 1
+	grep "git< version 1" log
 '
 
 
diff --git a/transport-helper.c b/transport-helper.c
index 143ca008c8..ac1937f1e1 100644
--- a/transport-helper.c
+++ b/transport-helper.c
@@ -105,6 +105,7 @@ static struct child_process *get_helper(struct transport *transport)
 {
 	struct helper_data *data = transport->data;
 	struct strbuf buf = STRBUF_INIT;
+	struct strbuf version_advert = STRBUF_INIT;
 	struct child_process *helper;
 	int duped;
 	int code;
@@ -127,6 +128,11 @@ static struct child_process *get_helper(struct transport *transport)
 		argv_array_pushf(&helper->env_array, "%s=%s",
 				 GIT_DIR_ENVIRONMENT, get_git_dir());
 
+	get_client_protocol_version_advertisement(&version_advert);
+	if (version_advert.len > 0)
+		argv_array_pushf(&helper->env_array, "%s=%s",
+				 GIT_PROTOCOL_ENVIRONMENT, version_advert.buf);
+
 	code = start_command(helper);
 	if (code < 0 && errno == ENOENT)
 		die(_("unable to find remote helper for '%s'"), data->name);
-- 
2.20.1.415.g653613c723-goog

Re: [PATCH v6 1/1] protocol: advertise multiple supported versions

[Jonathan Tan]

Thanks. Overall, one major point: I think that the declaration of
supported versions need to be transport-scoped (search this email for
"transport-scoped" for context). And one medium point: it might be
better for protocol.version to be the preferred and maximum version, not
only the preferred version. I have other minor points, which you can
read below.

> Currently the client advertises that it supports the wire protocol
> version set in the protocol.version config. However, not all services
> support the same set of protocol versions. For example, git-receive-pack
> supports v1 and v0, but not v2. 

That's true, except <see next paragraph>.

> If a client connects to git-receive-pack
> and requests v2, it will instead be downgraded to v0.

This is a bit misleading. The client never requests v2 - connect.c has
an override that changes it to v0 (as can be seen from the diff of this
patch) before the request takes place. If we were to create a custom
client that requests v2, the server would die with "support for protocol
v2 not implemented yet". (But maybe this is a bug - the server should
downgrade instead.)

> Other services,
> such as git-upload-archive, do not do any version negotiation checks.
> 
> This patch creates a protocol version registry. Individual client and
> server programs register all the protocol versions they support prior to
> communicating with a remote instance. Versions should be listed in
> preference order; the version specified in protocol.version will
> automatically be moved to the front of the registry.

It took me a while to understand what's going on, so let me try to
summarize:

 - The main issue is that send-pack doesn't support v2, so it would be
   incorrect for the client to advertise v2; this is currently fine
   because of the override in connect.c. A side issue is that the client
   advertises v2 for other things (e.g. archive), which currently works
   only because the server ignores them.

 - To solve both these issues, each command declares which protocols it
   supports.

 - Because we now have such a list, it is not too difficult to pass the
   entire list to the server, so we should do so. (Servers already
   support multiple versions.)

> The protocol version registry is passed to remote helpers via the
> GIT_PROTOCOL environment variable.

This is unfortunate to me, but I can't think of a better way to do it -
we do need to communicate the allowed version list somehow. A transport
option cannot be ignored by an old remote helper, but an envvar can.

Backwards/forwards compatibility notes:

 - If an old Git uses a new remote helper, no GIT_PROTOCOL is passed so
   the remote helper uses v0 regardless of what's configured in the
   repo. This should be fine.
 - If a new Git uses an old remote helper, what's configured gets used
   (unless it's for send-pack, in which case a version override occurs),
   regardless of which versions Git declares that it supports. I can
   envision some situations in which this would trip us up, but we are
   already in this exact situation anyway.

> Additionally, remove special cases around advertising version=0.
> Previously we avoided adding version advertisements to the client's
> initial connection request if the client wanted version=0. However,
> including these advertisements does not change the version negotiation
> behavior, so it's better to have simpler code. As a side effect, this
> means that client operations over SSH will always include a
> "SendEnv=GIT_PROTOCOL" option on the SSH command line.

HTTP too, I think?

Also, this means that the client will send different information to the
server now, but it seems fine (as far as I know, clients already send
Git version information anyway).

Overall, I would write the commit message this way:

    A Git client can communicate with a Git server through various
    commands, and for each command, in various protocol versions. Each
    repo can define protocol.version as 0, 1, or 2 for the protocol
    version that it prefers. But this integer is handled differently by
    each command: if communicating with "upload-pack", the integer is
    passed verbatim to be interpreted by the server; if communicating
    with "receive-pack", the integer is hardcoded to be overridden to 0
    or 1; and if communicating with anything else (e.g. "archive"), the
    integer is passed verbatim but ignored by the server.

    Handle each command the same way by having each client command
    declare which versions it supports, and whenever a command is
    invoked, send its list of versions to the server (instead of one
    single version, as is done currently). The protocol.version is now
    used as a hint to which version the client prefers; if it is present
    in the list, it is moved to the front, otherwise the list is sent
    unchanged.

    The server already supports multiple versions, but currently chooses
    the greatest. Make the server choose the first supported version
    instead.

    Because remote helpers now need this list of versions, pass the list
    of versions as the GIT_PROTOCOL environment variable. (New remote
    helpers will make use of this, and old remote helpers will use the
    repo configuration.)

    <special case about advertising version=0>

    <unnecessary externs>

> diff --git a/builtin/archive.c b/builtin/archive.c
> index e74f675390..8adb9f381b 100644
> --- a/builtin/archive.c
> +++ b/builtin/archive.c
> @@ -8,6 +8,7 @@
>  #include "transport.h"
>  #include "parse-options.h"
>  #include "pkt-line.h"
> +#include "protocol.h"
>  #include "sideband.h"
>  
>  static void create_output_file(const char *output_file)
> @@ -94,6 +95,8 @@ int cmd_archive(int argc, const char **argv, const char *prefix)
>  		OPT_END()
>  	};
>  
> +	register_allowed_protocol_version(protocol_v0);
> +
>  	argc = parse_options(argc, argv, prefix, local_opts, NULL,
>  			     PARSE_OPT_KEEP_ALL);
>  

What happens if a command never declares any version? It might be best
to explicitly allow such a case, and treat it as a single list of
protocol_v0.

> diff --git a/connect.c b/connect.c
> index 94547e5056..57266b6cec 100644
> --- a/connect.c
> +++ b/connect.c
> @@ -1046,7 +1046,7 @@ static enum ssh_variant determine_ssh_variant(const char *ssh_command,
>   */
>  static struct child_process *git_connect_git(int fd[2], char *hostandport,
>  					     const char *path, const char *prog,
> -					     enum protocol_version version,
> +					     const struct strbuf *version_advert,
>  					     int flags)
>  {
>  	struct child_process *conn;
> @@ -1084,12 +1084,9 @@ static struct child_process *git_connect_git(int fd[2], char *hostandport,
>  		    prog, path, 0,
>  		    target_host, 0);
>  
> -	/* If using a new version put that stuff here after a second null byte */
> -	if (version > 0) {
> -		strbuf_addch(&request, '\0');
> -		strbuf_addf(&request, "version=%d%c",
> -			    version, '\0');
> -	}
> +	/* Add version fields after a second null byte */
> +	strbuf_addch(&request, '\0');
> +	strbuf_addf(&request, "%s%c", version_advert->buf, '\0');
>  
>  	packet_write(fd[1], request.buf, request.len);

As expected, the version information for git:// is now written
differently.

> @@ -1104,14 +1101,13 @@ static struct child_process *git_connect_git(int fd[2], char *hostandport,
>   */
>  static void push_ssh_options(struct argv_array *args, struct argv_array *env,
>  			     enum ssh_variant variant, const char *port,
> -			     enum protocol_version version, int flags)
> +			     const struct strbuf *version_advert, int flags)
>  {
> -	if (variant == VARIANT_SSH &&
> -	    version > 0) {
> +	if (variant == VARIANT_SSH) {
>  		argv_array_push(args, "-o");
>  		argv_array_push(args, "SendEnv=" GIT_PROTOCOL_ENVIRONMENT);
> -		argv_array_pushf(env, GIT_PROTOCOL_ENVIRONMENT "=version=%d",
> -				 version);
> +		argv_array_pushf(env, GIT_PROTOCOL_ENVIRONMENT "=%s",
> +				 version_advert->buf);
>  	}
>  
>  	if (flags & CONNECT_IPV4) {

Likewise for ssh://.

[snip other ssh code]

> @@ -1226,16 +1223,10 @@ struct child_process *git_connect(int fd[2], const char *url,
>  {
>  	char *hostandport, *path;
>  	struct child_process *conn;
> +	struct strbuf version_advert = STRBUF_INIT;
>  	enum protocol protocol;
> -	enum protocol_version version = get_protocol_version_config();
>  
> -	/*
> -	 * NEEDSWORK: If we are trying to use protocol v2 and we are planning
> -	 * to perform a push, then fallback to v0 since the client doesn't know
> -	 * how to push yet using v2.
> -	 */
> -	if (version == protocol_v2 && !strcmp("git-receive-pack", prog))
> -		version = protocol_v0;
> +	get_client_protocol_version_advertisement(&version_advert);
>  
>  	/* Without this we cannot rely on waitpid() to tell
>  	 * what happened to our children.

It's great that this hardcoded override is now removed. I see below (in
another file) that the same occurs for http://.

> +void register_allowed_protocol_versions_from_env(void)
> +{
> +	const char *git_protocol = getenv(GIT_PROTOCOL_ENVIRONMENT);
> +	const char *version_str;
> +	struct string_list version_list = STRING_LIST_INIT_DUP;
> +	struct string_list_item *version;
> +
> +	if (!git_protocol)
> +		return;
> +
> +	string_list_split(&version_list, git_protocol, ':', -1);
> +	for_each_string_list_item(version, &version_list) {
> +		if (skip_prefix(version->string, "version=", &version_str))
> +			register_allowed_protocol_version(
> +				parse_protocol_version(version_str));

Probably worth BUG() if does not start with "version="?

> +	}
> +	string_list_clear(&version_list, 0);
> +}

[snip]

> +void get_client_protocol_version_advertisement(struct strbuf *advert)
> +{
> +	int i, tmp_nr = nr_allowed_versions;
> +	enum protocol_version *tmp_allowed_versions, config_version;
> +	strbuf_reset(advert);
> +
> +	version_registration_locked = 1;
> +
> +	config_version = get_protocol_version_config();
> +	if (config_version == protocol_v0) {
> +		strbuf_addstr(advert, "version=0");
> +		return;
> +	}

Why is protocol v0 special-cased like this?

> +	if (tmp_nr > 0) {
> +		ALLOC_ARRAY(tmp_allowed_versions, tmp_nr);
> +		copy_array(tmp_allowed_versions, allowed_versions, tmp_nr,
> +			   sizeof(enum protocol_version));
> +	} else {
> +		ALLOC_ARRAY(tmp_allowed_versions, 1);
> +		tmp_nr = 1;
> +		tmp_allowed_versions[0] = config_version;

In this "else" block, wouldn't this mean that if we forget to declare
any versions, we might send an unsupported version to the server? I
didn't check this, but it might happen if we invoked
transport_fetch_refs() without going through any of the builtins (which
might happen in the case of a lazy fetch in a partial clone).

> +	}
> +
> +	if (tmp_allowed_versions[0] != config_version)
> +		for (i = 1; i < nr_allowed_versions; i++)
> +			if (tmp_allowed_versions[i] == config_version) {
> +				SWAP(tmp_allowed_versions[0],
> +				     tmp_allowed_versions[i]);
> +			}

It doesn't seem right to just directly swap the config_version with the
front.

Also, I think this is more complicated that it needs to be. You could
just search for the config version; if found, loop backwards starting
from the index, shuffling to the right, then put the config version in
element 0.

> +
> +	strbuf_addf(advert, "version=%s",
> +		    format_protocol_version(tmp_allowed_versions[0]));
> +	for (i = 1; i < tmp_nr; i++)
> +		strbuf_addf(advert, ":version=%s",
> +			    format_protocol_version(tmp_allowed_versions[i]));
> +
> +	free(tmp_allowed_versions);
> +}

[snip]

>  	const char *git_protocol = getenv(GIT_PROTOCOL_ENVIRONMENT);
> @@ -38,9 +142,10 @@ enum protocol_version determine_protocol_version_server(void)
>  	/*
>  	 * Determine which protocol version the client has requested.  Since
>  	 * multiple 'version' keys can be sent by the client, indicating that
> -	 * the client is okay to speak any of them, select the greatest version
> -	 * that the client has requested.  This is due to the assumption that
> -	 * the most recent protocol version will be the most state-of-the-art.
> +	 * the client is okay to speak any of them, select the first
> +	 * recognizable version that the client has requested.  This is due to
> +	 * the assumption that the protocol versions will be listed in
> +	 * preference order.
>  	 */
>  	if (git_protocol) {
>  		struct string_list list = STRING_LIST_INIT_DUP;
> @@ -53,8 +158,11 @@ enum protocol_version determine_protocol_version_server(void)
>  
>  			if (skip_prefix(item->string, "version=", &value)) {
>  				v = parse_protocol_version(value);
> -				if (v > version)
> +				if (v != protocol_unknown_version &&
> +				    is_allowed_protocol_version(v)) {
>  					version = v;
> +					break;
> +				}
>  			}
>  		}

Here we see that the server now takes the first version, as discussed.

> diff --git a/protocol.h b/protocol.h
> index 2ad35e433c..88330fd0ee 100644
> --- a/protocol.h
> +++ b/protocol.h
> @@ -14,7 +14,24 @@ enum protocol_version {
>   * 'protocol.version' config.  If unconfigured, a value of 'protocol_v0' is
>   * returned.
>   */
> -extern enum protocol_version get_protocol_version_config(void);
> +enum protocol_version get_protocol_version_config(void);
> +
> +/*
> + * Register an allowable protocol version for a given operation. Registration
> + * must occur before attempting to advertise a version to a server process.
> + */
> +void register_allowed_protocol_version(enum protocol_version version);

I think the allowed protocol versions have to be transport-scoped. A
single builtin command may invoke multiple remote commands (as will be
the case if a lazy fetch in a partial clone is ever required).

> @@ -360,16 +373,10 @@ static struct discovery *discover_refs(const char *service, int for_push)
>  		strbuf_addf(&refs_url, "service=%s", service);
>  	}
>  
> -	/*
> -	 * NEEDSWORK: If we are trying to use protocol v2 and we are planning
> -	 * to perform a push, then fallback to v0 since the client doesn't know
> -	 * how to push yet using v2.
> -	 */
> -	if (version == protocol_v2 && !strcmp("git-receive-pack", service))
> -		version = protocol_v0;
> +	get_client_protocol_version_advertisement(&version_advert);
>  
>  	/* Add the extra Git-Protocol header */
> -	if (get_protocol_http_header(version, &protocol_header))
> +	if (get_client_protocol_http_header(&version_advert, &protocol_header))
>  		string_list_append(&extra_headers, protocol_header.buf);
>  
>  	memset(&http_options, 0, sizeof(http_options));

Great to see that this hardcoded override is also removed for http://.

> @@ -489,10 +489,8 @@ test_expect_success 'push with http:// and a config of v2 does not request v2' '
>  	git -C "$HTTPD_DOCUMENT_ROOT_PATH/http_parent" log -1 --format=%s client_branch >expect &&
>  	test_cmp expect actual &&
>  
> -	# Client didnt request to use protocol v2
> -	! grep "Git-Protocol: version=2" log &&
> -	# Server didnt respond using protocol v2
> -	! grep "git< version 2" log
> +	# Server responded with version 1
> +	grep "git< version 1" log
>  '

Change the test title - the client indeed would request 2, but since 1
is in front, the server will use version 1.

Re: [PATCH v6 1/1] protocol: advertise multiple supported versions

[Josh Steadmon]

On 2019.02.05 11:42, Jonathan Tan wrote:
> Thanks. Overall, one major point: I think that the declaration of
> supported versions need to be transport-scoped (search this email for
> "transport-scoped" for context). And one medium point: it might be
> better for protocol.version to be the preferred and maximum version, not
> only the preferred version. I have other minor points, which you can
> read below.

Thanks for the review, and apologies for the delayed reply. I have some
questions below about making the version advertisement transport scoped.
I think you're right about making the config version the maximum,
although I believe we've also avoided making the protocol versions
strictly orderable, so it may not be feasible if we want to keep that
feature.

> > Currently the client advertises that it supports the wire protocol
> > version set in the protocol.version config. However, not all services
> > support the same set of protocol versions. For example, git-receive-pack
> > supports v1 and v0, but not v2.
>
> That's true, except <see next paragraph>.
>
> > If a client connects to git-receive-pack
> > and requests v2, it will instead be downgraded to v0.
>
> This is a bit misleading. The client never requests v2 - connect.c has
> an override that changes it to v0 (as can be seen from the diff of this
> patch) before the request takes place. If we were to create a custom
> client that requests v2, the server would die with "support for protocol
> v2 not implemented yet". (But maybe this is a bug - the server should
> downgrade instead.)

Ack, will fix the description in the next version.


> > Other services,
> > such as git-upload-archive, do not do any version negotiation checks.
> >
> > This patch creates a protocol version registry. Individual client and
> > server programs register all the protocol versions they support prior to
> > communicating with a remote instance. Versions should be listed in
> > preference order; the version specified in protocol.version will
> > automatically be moved to the front of the registry.
>
> It took me a while to understand what's going on, so let me try to
> summarize:
>
>  - The main issue is that send-pack doesn't support v2, so it would be
>    incorrect for the client to advertise v2; this is currently fine
>    because of the override in connect.c. A side issue is that the client
>    advertises v2 for other things (e.g. archive), which currently works
>    only because the server ignores them.
>
>  - To solve both these issues, each command declares which protocols it
>    supports.
>
>  - Because we now have such a list, it is not too difficult to pass the
>    entire list to the server, so we should do so. (Servers already
>    support multiple versions.)
>
> > The protocol version registry is passed to remote helpers via the
> > GIT_PROTOCOL environment variable.
>
> This is unfortunate to me, but I can't think of a better way to do it -
> we do need to communicate the allowed version list somehow. A transport
> option cannot be ignored by an old remote helper, but an envvar can.
>
> Backwards/forwards compatibility notes:
>
>  - If an old Git uses a new remote helper, no GIT_PROTOCOL is passed so
>    the remote helper uses v0 regardless of what's configured in the
>    repo. This should be fine.
>  - If a new Git uses an old remote helper, what's configured gets used
>    (unless it's for send-pack, in which case a version override occurs),
>    regardless of which versions Git declares that it supports. I can
>    envision some situations in which this would trip us up, but we are
>    already in this exact situation anyway.
>
> > Additionally, remove special cases around advertising version=0.
> > Previously we avoided adding version advertisements to the client's
> > initial connection request if the client wanted version=0. However,
> > including these advertisements does not change the version negotiation
> > behavior, so it's better to have simpler code. As a side effect, this
> > means that client operations over SSH will always include a
> > "SendEnv=GIT_PROTOCOL" option on the SSH command line.
>
> HTTP too, I think?

Correct.


> Also, this means that the client will send different information to the
> server now, but it seems fine (as far as I know, clients already send
> Git version information anyway).
>
> Overall, I would write the commit message this way:
>
>     A Git client can communicate with a Git server through various
>     commands, and for each command, in various protocol versions. Each
>     repo can define protocol.version as 0, 1, or 2 for the protocol
>     version that it prefers. But this integer is handled differently by
>     each command: if communicating with "upload-pack", the integer is
>     passed verbatim to be interpreted by the server; if communicating
>     with "receive-pack", the integer is hardcoded to be overridden to 0
>     or 1; and if communicating with anything else (e.g. "archive"), the
>     integer is passed verbatim but ignored by the server.
>
>     Handle each command the same way by having each client command
>     declare which versions it supports, and whenever a command is
>     invoked, send its list of versions to the server (instead of one
>     single version, as is done currently). The protocol.version is now
>     used as a hint to which version the client prefers; if it is present
>     in the list, it is moved to the front, otherwise the list is sent
>     unchanged.
>
>     The server already supports multiple versions, but currently chooses
>     the greatest. Make the server choose the first supported version
>     instead.
>
>     Because remote helpers now need this list of versions, pass the list
>     of versions as the GIT_PROTOCOL environment variable. (New remote
>     helpers will make use of this, and old remote helpers will use the
>     repo configuration.)
>
>     <special case about advertising version=0>
>
>     <unnecessary externs>

Ack, will adjust the wording in the next version. Thanks for the
suggestion.


> > diff --git a/builtin/archive.c b/builtin/archive.c
> > index e74f675390..8adb9f381b 100644
> > --- a/builtin/archive.c
> > +++ b/builtin/archive.c
> > @@ -8,6 +8,7 @@
> >  #include "transport.h"
> >  #include "parse-options.h"
> >  #include "pkt-line.h"
> > +#include "protocol.h"
> >  #include "sideband.h"
> >
> >  static void create_output_file(const char *output_file)
> > @@ -94,6 +95,8 @@ int cmd_archive(int argc, const char **argv, const char *prefix)
> >             OPT_END()
> >     };
> >
> > +   register_allowed_protocol_version(protocol_v0);
> > +
> >     argc = parse_options(argc, argv, prefix, local_opts, NULL,
> >                          PARSE_OPT_KEEP_ALL);
> >
>
> What happens if a command never declares any version? It might be best
> to explicitly allow such a case, and treat it as a single list of
> protocol_v0.

Addressed below.


> > diff --git a/connect.c b/connect.c
> > index 94547e5056..57266b6cec 100644
> > --- a/connect.c
> > +++ b/connect.c
> > @@ -1046,7 +1046,7 @@ static enum ssh_variant determine_ssh_variant(const char *ssh_command,
> >   */
> >  static struct child_process *git_connect_git(int fd[2], char *hostandport,
> >                                          const char *path, const char *prog,
> > -                                        enum protocol_version version,
> > +                                        const struct strbuf *version_advert,
> >                                          int flags)
> >  {
> >     struct child_process *conn;
> > @@ -1084,12 +1084,9 @@ static struct child_process *git_connect_git(int fd[2], char *hostandport,
> >                 prog, path, 0,
> >                 target_host, 0);
> >
> > -   /* If using a new version put that stuff here after a second null byte */
> > -   if (version > 0) {
> > -           strbuf_addch(&request, '\0');
> > -           strbuf_addf(&request, "version=%d%c",
> > -                       version, '\0');
> > -   }
> > +   /* Add version fields after a second null byte */
> > +   strbuf_addch(&request, '\0');
> > +   strbuf_addf(&request, "%s%c", version_advert->buf, '\0');
> >
> >     packet_write(fd[1], request.buf, request.len);
>
> As expected, the version information for git:// is now written
> differently.
>
> > @@ -1104,14 +1101,13 @@ static struct child_process *git_connect_git(int fd[2], char *hostandport,
> >   */
> >  static void push_ssh_options(struct argv_array *args, struct argv_array *env,
> >                          enum ssh_variant variant, const char *port,
> > -                        enum protocol_version version, int flags)
> > +                        const struct strbuf *version_advert, int flags)
> >  {
> > -   if (variant == VARIANT_SSH &&
> > -       version > 0) {
> > +   if (variant == VARIANT_SSH) {
> >             argv_array_push(args, "-o");
> >             argv_array_push(args, "SendEnv=" GIT_PROTOCOL_ENVIRONMENT);
> > -           argv_array_pushf(env, GIT_PROTOCOL_ENVIRONMENT "=version=%d",
> > -                            version);
> > +           argv_array_pushf(env, GIT_PROTOCOL_ENVIRONMENT "=%s",
> > +                            version_advert->buf);
> >     }
> >
> >     if (flags & CONNECT_IPV4) {
>
> Likewise for ssh://.
>
> [snip other ssh code]
>
> > @@ -1226,16 +1223,10 @@ struct child_process *git_connect(int fd[2], const char *url,
> >  {
> >     char *hostandport, *path;
> >     struct child_process *conn;
> > +   struct strbuf version_advert = STRBUF_INIT;
> >     enum protocol protocol;
> > -   enum protocol_version version = get_protocol_version_config();
> >
> > -   /*
> > -    * NEEDSWORK: If we are trying to use protocol v2 and we are planning
> > -    * to perform a push, then fallback to v0 since the client doesn't know
> > -    * how to push yet using v2.
> > -    */
> > -   if (version == protocol_v2 && !strcmp("git-receive-pack", prog))
> > -           version = protocol_v0;
> > +   get_client_protocol_version_advertisement(&version_advert);
> >
> >     /* Without this we cannot rely on waitpid() to tell
> >      * what happened to our children.
>
> It's great that this hardcoded override is now removed. I see below (in
> another file) that the same occurs for http://.
>
> > +void register_allowed_protocol_versions_from_env(void)
> > +{
> > +   const char *git_protocol = getenv(GIT_PROTOCOL_ENVIRONMENT);
> > +   const char *version_str;
> > +   struct string_list version_list = STRING_LIST_INIT_DUP;
> > +   struct string_list_item *version;
> > +
> > +   if (!git_protocol)
> > +           return;
> > +
> > +   string_list_split(&version_list, git_protocol, ':', -1);
> > +   for_each_string_list_item(version, &version_list) {
> > +           if (skip_prefix(version->string, "version=", &version_str))
> > +                   register_allowed_protocol_version(
> > +                           parse_protocol_version(version_str));
>
> Probably worth BUG() if does not start with "version="?

Agreed.


> > +   }
> > +   string_list_clear(&version_list, 0);
> > +}
>
> [snip]
>
> > +void get_client_protocol_version_advertisement(struct strbuf *advert)
> > +{
> > +   int i, tmp_nr = nr_allowed_versions;
> > +   enum protocol_version *tmp_allowed_versions, config_version;
> > +   strbuf_reset(advert);
> > +
> > +   version_registration_locked = 1;
> > +
> > +   config_version = get_protocol_version_config();
> > +   if (config_version == protocol_v0) {
> > +           strbuf_addstr(advert, "version=0");
> > +           return;
> > +   }
>
> Why is protocol v0 special-cased like this?

To semi-preserve the existing behavior that no version negotiation would
happen when the config specifies version 0. Now we'll send out a version
advertisement where we wouldn't have before, but we only advertise v0 so
that no negotiation can happen.


> > +   if (tmp_nr > 0) {
> > +           ALLOC_ARRAY(tmp_allowed_versions, tmp_nr);
> > +           copy_array(tmp_allowed_versions, allowed_versions, tmp_nr,
> > +                      sizeof(enum protocol_version));
> > +   } else {
> > +           ALLOC_ARRAY(tmp_allowed_versions, 1);
> > +           tmp_nr = 1;
> > +           tmp_allowed_versions[0] = config_version;
>
> In this "else" block, wouldn't this mean that if we forget to declare
> any versions, we might send an unsupported version to the server? I
> didn't check this, but it might happen if we invoked
> transport_fetch_refs() without going through any of the builtins (which
> might happen in the case of a lazy fetch in a partial clone).

Hmm yeah. I changed the else{} here to die if we haven't registered any
versions prior to creating the advertisement string. t0410 case 9 and
t5702 case 20 both fail, and both are lazy fetches in partial clones.

In this case, defaulting to the version in the config works, because
fetching is valid for all protocol versions. But if we ever add some
version X where fetching hasn't been implemented this would break.

Should we change the else{} case here to be a BUG() and then fix
fetch-object to register its supported versions?


> > +   }
> > +
> > +   if (tmp_allowed_versions[0] != config_version)
> > +           for (i = 1; i < nr_allowed_versions; i++)
> > +                   if (tmp_allowed_versions[i] == config_version) {
> > +                           SWAP(tmp_allowed_versions[0],
> > +                                tmp_allowed_versions[i]);
> > +                   }
>
> It doesn't seem right to just directly swap the config_version with the
> front.
>
> Also, I think this is more complicated that it needs to be. You could
> just search for the config version; if found, loop backwards starting
> from the index, shuffling to the right, then put the config version in
> element 0.

Ack. Will fix.


> > +
> > +   strbuf_addf(advert, "version=%s",
> > +               format_protocol_version(tmp_allowed_versions[0]));
> > +   for (i = 1; i < tmp_nr; i++)
> > +           strbuf_addf(advert, ":version=%s",
> > +                       format_protocol_version(tmp_allowed_versions[i]));
> > +
> > +   free(tmp_allowed_versions);
> > +}
>
> [snip]
>
> >     const char *git_protocol = getenv(GIT_PROTOCOL_ENVIRONMENT);
> > @@ -38,9 +142,10 @@ enum protocol_version determine_protocol_version_server(void)
> >     /*
> >      * Determine which protocol version the client has requested.  Since
> >      * multiple 'version' keys can be sent by the client, indicating that
> > -    * the client is okay to speak any of them, select the greatest version
> > -    * that the client has requested.  This is due to the assumption that
> > -    * the most recent protocol version will be the most state-of-the-art.
> > +    * the client is okay to speak any of them, select the first
> > +    * recognizable version that the client has requested.  This is due to
> > +    * the assumption that the protocol versions will be listed in
> > +    * preference order.
> >      */
> >     if (git_protocol) {
> >             struct string_list list = STRING_LIST_INIT_DUP;
> > @@ -53,8 +158,11 @@ enum protocol_version determine_protocol_version_server(void)
> > 
> >                     if (skip_prefix(item->string, "version=", &value)) {
> >                             v = parse_protocol_version(value);
> > -                           if (v > version)
> > +                           if (v != protocol_unknown_version &&
> > +                               is_allowed_protocol_version(v)) {
> >                                     version = v;
> > +                                   break;
> > +                           }
> >                     }
> >             }
>
> Here we see that the server now takes the first version, as discussed.
>
> > diff --git a/protocol.h b/protocol.h
> > index 2ad35e433c..88330fd0ee 100644
> > --- a/protocol.h
> > +++ b/protocol.h
> > @@ -14,7 +14,24 @@ enum protocol_version {
> >   * 'protocol.version' config.  If unconfigured, a value of 'protocol_v0' is
> >   * returned.
> >   */
> > -extern enum protocol_version get_protocol_version_config(void);
> > +enum protocol_version get_protocol_version_config(void);
> > +
> > +/*
> > + * Register an allowable protocol version for a given operation. Registration
> > + * must occur before attempting to advertise a version to a server process.
> > + */
> > +void register_allowed_protocol_version(enum protocol_version version);
>
> I think the allowed protocol versions have to be transport-scoped. A
> single builtin command may invoke multiple remote commands (as will be
> the case if a lazy fetch in a partial clone is ever required).

Wouldn't it need to be per-transport per-command then? For example, if
we ever added a hypothetical git-fetch-then-rebase-then-push builtin, we
couldn't use the same version advertisement for the fetch and the push,
even if they're still using the same transport. Or would we have to use
separate transport objects for the fetch and the push in such a
situation?

If we do move the version list into the transport, what would be the
right way to register the allowed versions? Maybe we would make each
vtable entry declare its versions?

> > @@ -360,16 +373,10 @@ static struct discovery *discover_refs(const char *service, int for_push)
> >             strbuf_addf(&refs_url, "service=%s", service);
> >     }
> > 
> > -   /*
> > -    * NEEDSWORK: If we are trying to use protocol v2 and we are planning
> > -    * to perform a push, then fallback to v0 since the client doesn't know
> > -    * how to push yet using v2.
> > -    */
> > -   if (version == protocol_v2 && !strcmp("git-receive-pack", service))
> > -           version = protocol_v0;
> > +   get_client_protocol_version_advertisement(&version_advert);
> > 
> >     /* Add the extra Git-Protocol header */
> > -   if (get_protocol_http_header(version, &protocol_header))
> > +   if (get_client_protocol_http_header(&version_advert, &protocol_header))
> >             string_list_append(&extra_headers, protocol_header.buf);
> > 
> >     memset(&http_options, 0, sizeof(http_options));
>
> Great to see that this hardcoded override is also removed for http://.
>
> > @@ -489,10 +489,8 @@ test_expect_success 'push with http:// and a config of v2 does not request v2' '
> >     git -C "$HTTPD_DOCUMENT_ROOT_PATH/http_parent" log -1 --format=%s client_branch >expect &&
> >     test_cmp expect actual &&
> > 
> > -   # Client didnt request to use protocol v2
> > -   ! grep "Git-Protocol: version=2" log &&
> > -   # Server didnt respond using protocol v2
> > -   ! grep "git< version 2" log
> > +   # Server responded with version 1
> > +   grep "git< version 1" log
> >  '
>
> Change the test title - the client indeed would request 2, but since 1
> is in front, the server will use version 1.

Will do.

Re: [PATCH v6 1/1] protocol: advertise multiple supported versions

[Jonathan Tan]

> > > +void get_client_protocol_version_advertisement(struct strbuf *advert)
> > > +{
> > > +   int i, tmp_nr = nr_allowed_versions;
> > > +   enum protocol_version *tmp_allowed_versions, config_version;
> > > +   strbuf_reset(advert);
> > > +
> > > +   version_registration_locked = 1;
> > > +
> > > +   config_version = get_protocol_version_config();
> > > +   if (config_version == protocol_v0) {
> > > +           strbuf_addstr(advert, "version=0");
> > > +           return;
> > > +   }
> >
> > Why is protocol v0 special-cased like this?
> 
> To semi-preserve the existing behavior that no version negotiation would
> happen when the config specifies version 0. Now we'll send out a version
> advertisement where we wouldn't have before, but we only advertise v0 so
> that no negotiation can happen.

Ah, I see.

This might be an argument for deciding that the protocol versions are
strictly orderable. I don't recall any discussions around this, but it
makes sense to me that someone specifying version 1 would be OK with
version 0 but not version 2, and someone specifying version 2 would be
OK with any of 0, 1, and 2. So I'm OK with making it effectively
strictly orderable by introducing the concept of a maximum.

And if we have this concept of a maximum, then we can have the
no-negotiation-if-v0-configured behavior without any special cases.

> > > +   if (tmp_nr > 0) {
> > > +           ALLOC_ARRAY(tmp_allowed_versions, tmp_nr);
> > > +           copy_array(tmp_allowed_versions, allowed_versions, tmp_nr,
> > > +                      sizeof(enum protocol_version));
> > > +   } else {
> > > +           ALLOC_ARRAY(tmp_allowed_versions, 1);
> > > +           tmp_nr = 1;
> > > +           tmp_allowed_versions[0] = config_version;
> >
> > In this "else" block, wouldn't this mean that if we forget to declare
> > any versions, we might send an unsupported version to the server? I
> > didn't check this, but it might happen if we invoked
> > transport_fetch_refs() without going through any of the builtins (which
> > might happen in the case of a lazy fetch in a partial clone).
> 
> Hmm yeah. I changed the else{} here to die if we haven't registered any
> versions prior to creating the advertisement string. t0410 case 9 and
> t5702 case 20 both fail, and both are lazy fetches in partial clones.
> 
> In this case, defaulting to the version in the config works, because
> fetching is valid for all protocol versions. But if we ever add some
> version X where fetching hasn't been implemented this would break.
> 
> Should we change the else{} case here to be a BUG() and then fix
> fetch-object to register its supported versions?

Yes, I think so.

> > > diff --git a/protocol.h b/protocol.h
> > > index 2ad35e433c..88330fd0ee 100644
> > > --- a/protocol.h
> > > +++ b/protocol.h
> > > @@ -14,7 +14,24 @@ enum protocol_version {
> > >   * 'protocol.version' config.  If unconfigured, a value of 'protocol_v0' is
> > >   * returned.
> > >   */
> > > -extern enum protocol_version get_protocol_version_config(void);
> > > +enum protocol_version get_protocol_version_config(void);
> > > +
> > > +/*
> > > + * Register an allowable protocol version for a given operation. Registration
> > > + * must occur before attempting to advertise a version to a server process.
> > > + */
> > > +void register_allowed_protocol_version(enum protocol_version version);
> >
> > I think the allowed protocol versions have to be transport-scoped. A
> > single builtin command may invoke multiple remote commands (as will be
> > the case if a lazy fetch in a partial clone is ever required).
> 
> Wouldn't it need to be per-transport per-command then? For example, if
> we ever added a hypothetical git-fetch-then-rebase-then-push builtin, we
> couldn't use the same version advertisement for the fetch and the push,
> even if they're still using the same transport. Or would we have to use
> separate transport objects for the fetch and the push in such a
> situation?

If we make each implementation of the vtable function read the config
and compute its own list of allowed protocols (is this what you mean by
"make each vtable entry declare its versions" below?), then we still
leave open the possibility of using the same transport object for fetch
and push.

> If we do move the version list into the transport, what would be the
> right way to register the allowed versions? Maybe we would make each
> vtable entry declare its versions?

This makes sense to me. I think that the implementations of
transport_vtable.connect might need to take in a list of versions, but
fetch and push can probably read the config by themselves so the API
does not need to change.